2020 |
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 | Alzate-Carvajal, Natalia; Luican-Mayer, Adina Functionalized Graphene Surfaces for Selective Gas Sensing Journal Article ACS Omega, 2020, ISBN: 2470-1343. @article{Alzate-Carvajal2020, title = {Functionalized Graphene Surfaces for Selective Gas Sensing}, author = { Natalia Alzate-Carvajal and Adina Luican-Mayer }, url = {https://pubs.acs.org/doi/10.1021/acsomega.0c02861}, doi = {10.1021/acsomega.0c02861}, isbn = {2470-1343}, year = {2020}, date = {2020-08-21}, journal = {ACS Omega}, abstract = {Environmental monitoring through gas sensors is paramount for the safety and security of industrial workers and for ecological protection. Graphene is among the most promising materials considered for next-generation gas sensing due to its properties such as mechanical strength and flexibility, high surface-to-volume ratio, large conductivity, and low electrical noise. While gas sensors based on graphene devices have already demonstrated high sensitivity, one of the most important figures of merit, selectivity, remains a challenge. In the past few years, however, surface functionalization emerged as a potential route to achieve selectivity. This review surveys the recent advances in the fabrication and characterization of graphene and reduced graphene oxide gas sensors chemically functionalized with aromatic molecules and polymers with the goal of improving selectivity toward specific gases as well as overall sensor performance.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Environmental monitoring through gas sensors is paramount for the safety and security of industrial workers and for ecological protection. Graphene is among the most promising materials considered for next-generation gas sensing due to its properties such as mechanical strength and flexibility, high surface-to-volume ratio, large conductivity, and low electrical noise. While gas sensors based on graphene devices have already demonstrated high sensitivity, one of the most important figures of merit, selectivity, remains a challenge. In the past few years, however, surface functionalization emerged as a potential route to achieve selectivity. This review surveys the recent advances in the fabrication and characterization of graphene and reduced graphene oxide gas sensors chemically functionalized with aromatic molecules and polymers with the goal of improving selectivity toward specific gases as well as overall sensor performance. |
 | Rautela, Ranjana; Scarfe, Samantha; Guay, Jean-Michel; Lazar, Petr; Pykal, Martin; Azimi, Saied; Grenapin, Cedric; Boddison-Chouinard, Justin; Halpin, Alexei; Wang, Weixiang; Andrzejewski, Lukasz; Plumadore, Ryan; Park, Jeongwon; Menard, Jean-Michel; Otyepka, Michal; Luican-Mayer, Adina Mechanistic insight into the limiting factors of graphene-based environmental sensors Journal Article ACS Applied Materials & Interfaces, 2020, ISSN: 1944-8244. @article{Rautela2020, title = {Mechanistic insight into the limiting factors of graphene-based environmental sensors}, author = {Ranjana Rautela and Samantha Scarfe and Jean-Michel Guay and Petr Lazar and Martin Pykal and Saied Azimi and Cedric Grenapin and Justin Boddison-Chouinard and Alexei Halpin and Weixiang Wang and Lukasz Andrzejewski and Ryan Plumadore and Jeongwon Park and Jean-Michel Menard and Michal Otyepka and Adina Luican-Mayer}, url = {https://doi.org/10.1021/acsami.0c09051}, doi = {10.1021/acsami.0c09051}, issn = {1944-8244}, year = {2020}, date = {2020-07-13}, journal = {ACS Applied Materials & Interfaces}, publisher = {American Chemical Society}, abstract = {Graphene has demonstrated great promise for technological use, yet control over material growth and understanding of how material imperfections affect the performance of devices are challenges that hamper the development of applications. In this work we reveal new insight into the connections between the performance of the graphene devices as environmental sensors and the microscopic details of the interactions at the sensing surface. We monitor changes in the resistance of the chemical-vapour deposition grown graphene devices as exposed to different concentrations of ethanol. We perform thermal surface treatments after the devices are fabricated, use scanning probe microscopy to visualize their effects down to nanometer scale and correlate them with the measured performance of the device as an ethanol sensor. Our observations are compared to theoretical calculations of charge transfers between molecules and the graphene surface. We find that, although often overlooked, the surface cleanliness after device fabrication is responsible for the device performance and reliability. These results further our understanding of the mechanisms of sensing in graphene-based environmental sensors and pave the way to optimizing such devices, especially for their miniaturization, as with decrising size of the active zone the potential role of contaminants will rise.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Graphene has demonstrated great promise for technological use, yet control over material growth and understanding of how material imperfections affect the performance of devices are challenges that hamper the development of applications. In this work we reveal new insight into the connections between the performance of the graphene devices as environmental sensors and the microscopic details of the interactions at the sensing surface. We monitor changes in the resistance of the chemical-vapour deposition grown graphene devices as exposed to different concentrations of ethanol. We perform thermal surface treatments after the devices are fabricated, use scanning probe microscopy to visualize their effects down to nanometer scale and correlate them with the measured performance of the device as an ethanol sensor. Our observations are compared to theoretical calculations of charge transfers between molecules and the graphene surface. We find that, although often overlooked, the surface cleanliness after device fabrication is responsible for the device performance and reliability. These results further our understanding of the mechanisms of sensing in graphene-based environmental sensors and pave the way to optimizing such devices, especially for their miniaturization, as with decrising size of the active zone the potential role of contaminants will rise. |
 | Plumadore, Ryan; Ezzi, Mohammed Al M; Adam, Shaffique; Luican-Mayer, Adina Moiré patterns in graphene–rhenium disulfide vertical heterostructures Journal Article Journal of Applied Physics, 128 (4), pp. 044303, 2020. @article{doi:10.1063/5.0015643, title = {Moiré patterns in graphene–rhenium disulfide vertical heterostructures}, author = {Ryan Plumadore and Mohammed M Al Ezzi and Shaffique Adam and Adina Luican-Mayer}, url = {https://doi.org/10.1063/5.0015643}, doi = {10.1063/5.0015643}, year = {2020}, date = {2020-01-01}, journal = {Journal of Applied Physics}, volume = {128}, number = {4}, pages = {044303}, abstract = {Vertical stacking of atomically thin materials offers a large platform for realizing novel properties enabled by proximity effects and moiré patterns. Here, we focus on mechanically assembled heterostructures of graphene and ReS2, a van der Waals layered semiconductor. Using scanning tunneling microscopy and spectroscopy, we image the sharp edge between the two materials as well as areas of overlap. Locally resolved topographic images revealed the presence of a striped superpattern originating in the interlayer interactions between graphene's hexagonal structure and the triclinic, low in-plane symmetry of ReS2. We compare the results with a theoretical model that estimates the shape and angle dependence of the moiré pattern between graphene and ReS2. These results shed light on the complex interface phenomena between van der Waals materials with different lattice symmetries.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Vertical stacking of atomically thin materials offers a large platform for realizing novel properties enabled by proximity effects and moiré patterns. Here, we focus on mechanically assembled heterostructures of graphene and ReS2, a van der Waals layered semiconductor. Using scanning tunneling microscopy and spectroscopy, we image the sharp edge between the two materials as well as areas of overlap. Locally resolved topographic images revealed the presence of a striped superpattern originating in the interlayer interactions between graphene's hexagonal structure and the triclinic, low in-plane symmetry of ReS2. We compare the results with a theoretical model that estimates the shape and angle dependence of the moiré pattern between graphene and ReS2. These results shed light on the complex interface phenomena between van der Waals materials with different lattice symmetries. |
2019 |
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 | Boddison-Chouinard, Justin; Scarfe, Samantha; Watanabe, K; Taniguchi, T; Luican-Mayer, Adina Flattening van der Waals heterostructure interfaces by local thermal treatment Journal Article Applied Physics Letters, 115 (23), pp. 231603, 2019. @article{boddison2019flattening, title = {Flattening van der Waals heterostructure interfaces by local thermal treatment}, author = {Justin Boddison-Chouinard and Samantha Scarfe and K Watanabe and T Taniguchi and Adina Luican-Mayer}, url = {https://doi.org/10.1063/1.5131022}, doi = {10.1063/1.5131022}, year = {2019}, date = {2019-01-01}, journal = {Applied Physics Letters}, volume = {115}, number = {23}, pages = {231603}, publisher = {AIP Publishing LLC}, abstract = {Fabrication of custom-built heterostructures based on stacked 2D materials provides an effective method to controllably tune electronic and optical properties. To that end, optimizing fabrication techniques for building these heterostructures is imperative. A common challenge in layer-by-layer assembly of 2D materials is the formation of bubbles at atomically thin interfaces. We propose a technique for addressing this issue by removing the bubbles formed at the heterostructure interface in a custom-defined area using the heat generated by a laser equipped with raster scanning capabilities. We demonstrate that the density of bubbles formed at graphene-ReS2 interfaces can be controllably reduced using this method. We discuss an understanding of the flattening mechanism by considering the interplay of interface thermal conductivities and adhesion energies between two atomically thin 2D materials. The authors acknowledge funding from the National Sciences and Engineering Research Council (NSERC) Discovery Grant No. RGPIN-2016-06717. We also acknowledge the support of the Natural Sciences and Engineering Research Council of Canada (NSERC) through Strategic Project No. STPGP 521420 (Quantum Circuits in 2D materials). Growth of hexagonal boron nitride crystals (K.W. and T.T.) was supported by the Elemental Strategy Initiative conducted by the MEXT, Japan, and the CREST (No. JPMJCR15F3), J.S.T. We thank Professor Jean-Michel Menard and Professor Rui Huang for useful discussions and Emmanuelle Launay for technical support.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Fabrication of custom-built heterostructures based on stacked 2D materials provides an effective method to controllably tune electronic and optical properties. To that end, optimizing fabrication techniques for building these heterostructures is imperative. A common challenge in layer-by-layer assembly of 2D materials is the formation of bubbles at atomically thin interfaces. We propose a technique for addressing this issue by removing the bubbles formed at the heterostructure interface in a custom-defined area using the heat generated by a laser equipped with raster scanning capabilities. We demonstrate that the density of bubbles formed at graphene-ReS2 interfaces can be controllably reduced using this method. We discuss an understanding of the flattening mechanism by considering the interplay of interface thermal conductivities and adhesion energies between two atomically thin 2D materials. The authors acknowledge funding from the National Sciences and Engineering Research Council (NSERC) Discovery Grant No. RGPIN-2016-06717. We also acknowledge the support of the Natural Sciences and Engineering Research Council of Canada (NSERC) through Strategic Project No. STPGP 521420 (Quantum Circuits in 2D materials). Growth of hexagonal boron nitride crystals (K.W. and T.T.) was supported by the Elemental Strategy Initiative conducted by the MEXT, Japan, and the CREST (No. JPMJCR15F3), J.S.T. We thank Professor Jean-Michel Menard and Professor Rui Huang for useful discussions and Emmanuelle Launay for technical support. |
 | Luican-Mayer, Adina; Zhang, Yuan; DiLullo, Andrew; Li, Yang; Fisher, Brandon; Ulloa, Sergio E; Hla, Saw-Wai Negative differential resistance observed on the charge density wave of a transition metal dichalcogenide Journal Article Nanoscale, 11 (46), pp. 22351–22358, 2019. @article{luican2019negative, title = {Negative differential resistance observed on the charge density wave of a transition metal dichalcogenide}, author = {Adina Luican-Mayer and Yuan Zhang and Andrew DiLullo and Yang Li and Brandon Fisher and Sergio E Ulloa and Saw-Wai Hla}, url = {http://dx.doi.org/10.1039/C9NR07857F}, doi = {10.1039/C9NR07857F}, year = {2019}, date = {2019-01-01}, journal = {Nanoscale}, volume = {11}, number = {46}, pages = {22351--22358}, publisher = {Royal Society of Chemistry}, abstract = {Charge density waves and negative differential resistance are seemingly unconnected physical phenomena. The former is an ordered quantum fluid of electrons{,} intensely investigated for its relation with superconductivity{,} while the latter receives much attention for its potential applications in electronics. Here we show that these two phenomena can not only coexist but also that the localized electronic states of the charge density wave are essential to induce negative differential resistance in a transition metal dichalcogenide{,} 1T-TaS2. Using scanning tunneling microscopy and spectroscopy{,} we report the observation of negative differential resistance in the commensurate charge density wave state of 1T-TaS2. The observed phenomenon is explained by the interplay of interlayer and intra-layer tunneling with the participation of the atomically localized states of the charge density wave maxima and minima. We demonstrate that lattice defects can locally affect the coupling between the layers and are therefore a mechanism to realize NDR in these materials.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Charge density waves and negative differential resistance are seemingly unconnected physical phenomena. The former is an ordered quantum fluid of electrons{,} intensely investigated for its relation with superconductivity{,} while the latter receives much attention for its potential applications in electronics. Here we show that these two phenomena can not only coexist but also that the localized electronic states of the charge density wave are essential to induce negative differential resistance in a transition metal dichalcogenide{,} 1T-TaS2. Using scanning tunneling microscopy and spectroscopy{,} we report the observation of negative differential resistance in the commensurate charge density wave state of 1T-TaS2. The observed phenomenon is explained by the interplay of interlayer and intra-layer tunneling with the participation of the atomically localized states of the charge density wave maxima and minima. We demonstrate that lattice defects can locally affect the coupling between the layers and are therefore a mechanism to realize NDR in these materials. |
 | Ramos, Sergio LLM; Plumadore, Ryan; Boddison-Chouinard, Justin; Hla, Saw Wai; Guest, Jeffrey R; Gosztola, David J; Pimenta, Marcos A; Luican-Mayer, Adina Suppression of the commensurate charge density wave phase in ultrathin 1 T- TaS 2 evidenced by Raman hyperspectral analysis Journal Article Physical Review B, 100 (16), pp. 165414, 2019. @article{ramos2019suppression, title = {Suppression of the commensurate charge density wave phase in ultrathin 1 T- TaS 2 evidenced by Raman hyperspectral analysis}, author = {Sergio LLM Ramos and Ryan Plumadore and Justin Boddison-Chouinard and Saw Wai Hla and Jeffrey R Guest and David J Gosztola and Marcos A Pimenta and Adina Luican-Mayer}, url = {https://link.aps.org/doi/10.1103/PhysRevB.100.165414}, doi = {10.1103/PhysRevB.100.165414}, year = {2019}, date = {2019-01-01}, journal = {Physical Review B}, volume = {100}, number = {16}, pages = {165414}, publisher = {APS}, abstract = {Using temperature-dependent and low-frequency Raman spectroscopy, we address the question of how the transition from bulk to a few atomic layers affects the charge density wave (CDW) phases in 1T−TaS2. We find that for crystals with thickness larger than ≈10 nm the transition temperatures between the different phases as well as the hysteresis that occurs in the thermal cycle correspond to the ones expected for a bulk sample. However, when the crystals become thinner than ≈10 nm, the low-temperature commensurate CDW phases can be suppressed down to the experimentally accessible temperatures (∼80 K) upon cooling at moderate rates (∼5Kmin−1). In addition, even the near commensurate CDW phase is not accessible in few-layer flakes below ≈4 nm for even slower cooling rates (∼1Kmin−1). We employ Raman hyperspectral imaging to statistically confirm these findings and consider the interlayer coupling and its dynamics to play significant roles in determining the properties of CDW systems consisting of a few unit cells in the vertical direction.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Using temperature-dependent and low-frequency Raman spectroscopy, we address the question of how the transition from bulk to a few atomic layers affects the charge density wave (CDW) phases in 1T−TaS2. We find that for crystals with thickness larger than ≈10 nm the transition temperatures between the different phases as well as the hysteresis that occurs in the thermal cycle correspond to the ones expected for a bulk sample. However, when the crystals become thinner than ≈10 nm, the low-temperature commensurate CDW phases can be suppressed down to the experimentally accessible temperatures (∼80 K) upon cooling at moderate rates (∼5Kmin−1). In addition, even the near commensurate CDW phase is not accessible in few-layer flakes below ≈4 nm for even slower cooling rates (∼1Kmin−1). We employ Raman hyperspectral imaging to statistically confirm these findings and consider the interlayer coupling and its dynamics to play significant roles in determining the properties of CDW systems consisting of a few unit cells in the vertical direction. |
 | Stecher, Kristen; Huang, Symphony Hsiao-Yuan; Escorcio, Rodrigo; Luican-Mayer, Adina Demonstrating the concepts of sheet resistance, field effect, and mobility of a semiconductor using graphene field effect transistors Journal Article European Journal of Physics, 40 (6), pp. 065501, 2019. @article{stecher2019demonstrating, title = {Demonstrating the concepts of sheet resistance, field effect, and mobility of a semiconductor using graphene field effect transistors}, author = {Kristen Stecher and Symphony Hsiao-Yuan Huang and Rodrigo Escorcio and Adina Luican-Mayer}, url = {https://doi.org/10.1088%2F1361-6404%2Fab4444}, doi = {10.1088/1361-6404/ab4444}, year = {2019}, date = {2019-01-01}, journal = {European Journal of Physics}, volume = {40}, number = {6}, pages = {065501}, publisher = {IOP Publishing}, abstract = {With rapid advancement in the discovery of innovative materials, there is great opportunity for enhancing undergraduate laboratories teaching basic physics. Here, we exemplify a laboratory focused on basic concepts of semiconductors (sheet resistance, field effect, mobility) taking advantage of commercially available graphene devices, a cutting-edge material system. We present an experimental set-up and protocol that can be used to implement such a laboratory lecture. We introduce the physics concepts that will be explored and we discuss the results obtained when we carried out the described laboratory experiment.}, keywords = {}, pubstate = {published}, tppubtype = {article} } With rapid advancement in the discovery of innovative materials, there is great opportunity for enhancing undergraduate laboratories teaching basic physics. Here, we exemplify a laboratory focused on basic concepts of semiconductors (sheet resistance, field effect, mobility) taking advantage of commercially available graphene devices, a cutting-edge material system. We present an experimental set-up and protocol that can be used to implement such a laboratory lecture. We introduce the physics concepts that will be explored and we discuss the results obtained when we carried out the described laboratory experiment. |
 | Luican-Mayer, Adina A needle in a moiré stack Journal Article Nature Physics, 15 (11), pp. 1107–1108, 2019. @article{luican2019needle, title = {A needle in a moiré stack}, author = {Adina Luican-Mayer}, url = {https://doi.org/10.1038/s41567-019-0645-y}, doi = {10.1038/s41567-019-0645-y}, year = {2019}, date = {2019-01-01}, journal = {Nature Physics}, volume = {15}, number = {11}, pages = {1107--1108}, publisher = {Nature Publishing Group}, abstract = {Spatially resolved measurements of twisted bilayer graphene reveal more details of the strongly correlated electrons.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Spatially resolved measurements of twisted bilayer graphene reveal more details of the strongly correlated electrons. |
 | Boddison-Chouinard, Justin; Plumadore, Ryan; Luican-Mayer, Adina Fabricating van der Waals Heterostructures with Precise Rotational Alignment Journal Article JoVE (Journal of Visualized Experiments), (149), pp. e59727, 2019. @article{boddison2019fabricating, title = {Fabricating van der Waals Heterostructures with Precise Rotational Alignment}, author = {Justin Boddison-Chouinard and Ryan Plumadore and Adina Luican-Mayer}, url = {https://europepmc.org/article/med/31329181}, doi = {10.3791/59727}, year = {2019}, date = {2019-01-01}, journal = {JoVE (Journal of Visualized Experiments)}, number = {149}, pages = {e59727}, abstract = {In this work we describe a technique for creating new crystals (van der Waals heterostructures) by stacking distinct ultrathin layered 2D materials. We demonstrate not only lateral control but, importantly, also control over the angular alignment of adjacent layers. The core of the technique is represented by a home-built transfer setup which allows the user to control the position of the individual crystals involved in the transfer. This is achieved with sub-micrometer (translational) and sub-degree (angular) precision. Prior to stacking them together, the isolated crystals are individually manipulated by custom-designed moving stages that are controlled by a programmed software interface. Moreover, since the entire transfer setup is computer controlled, the user can remotely create precise heterostructures without coming into direct contact with the transfer setup, labeling this technique as "hands-free". In addition to presenting the transfer set-up, we also describe two techniques for preparing the crystals that are subsequently stacked.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In this work we describe a technique for creating new crystals (van der Waals heterostructures) by stacking distinct ultrathin layered 2D materials. We demonstrate not only lateral control but, importantly, also control over the angular alignment of adjacent layers. The core of the technique is represented by a home-built transfer setup which allows the user to control the position of the individual crystals involved in the transfer. This is achieved with sub-micrometer (translational) and sub-degree (angular) precision. Prior to stacking them together, the isolated crystals are individually manipulated by custom-designed moving stages that are controlled by a programmed software interface. Moreover, since the entire transfer setup is computer controlled, the user can remotely create precise heterostructures without coming into direct contact with the transfer setup, labeling this technique as "hands-free". In addition to presenting the transfer set-up, we also describe two techniques for preparing the crystals that are subsequently stacked. |
 | Guay, Jean-Michel ; Rautela, Ranjana ; Scarfe, Samantha ; Lazar, Petr ; Azimi, Saied ; Grenapin, Cedric ; Halpin, Alexei ; Wang, Weixang ; Andrzejewski, Lukasz ; Plumadore, Ryan ; Park, Jeongwon ; Otyepka, Michal ; Menard, Jean-Michel ; Luican-Mayer, Adina Mechanistic insight into the limiting factors of graphene-based environmental sensors Journal Article arXiv e-prints, pp. arXiv:1911.05757, 2019. @article{2019arXiv191105757G, title = {Mechanistic insight into the limiting factors of graphene-based environmental sensors}, author = {Jean-Michel {Guay} and Ranjana {Rautela} and Samantha {Scarfe} and Petr {Lazar} and Saied {Azimi} and Cedric {Grenapin} and Alexei {Halpin} and Weixang {Wang} and Lukasz {Andrzejewski} and Ryan {Plumadore} and Jeongwon {Park} and Michal {Otyepka} and Jean-Michel {Menard} and Adina {Luican-Mayer}}, url = {https://arxiv.org/abs/1911.05757}, year = {2019}, date = {2019-01-01}, journal = {arXiv e-prints}, pages = {arXiv:1911.05757}, abstract = {Graphene has demonstrated great promise for technological use, yet control over material growth and understanding of how material imperfections affect the performance of devices are challenges that hamper the development of applications. In this work we reveal new insight into the connections between the performance of the graphene devices as environmental sensors and the microscopic details of the interactions at the sensing surface. Specifically, we monitor changes in the resistance of the chemical-vapour deposition grown graphene devices as exposed to different concentrations of ethanol. We perform thermal surface treatments after the devices are fabricated, use scanning probe microscopy to visualize their effects on the graphene sensing surface down to nanometer scale and correlate them with the measured performance of the device as an ethanol sensor. Our observations are compared to theoretical calculations of charge transfers between molecules and the graphene surface. We find that, although often overlooked, the surface cleanliness after device fabrication is responsible for the device performance and reliability. These results further our understanding of the mechanisms of sensing in graphene-based environmental sensors and pave the way to optimizing such devices, especially for their miniaturization, as with decreasing size of the active zone the potential role of contaminants will rise.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Graphene has demonstrated great promise for technological use, yet control over material growth and understanding of how material imperfections affect the performance of devices are challenges that hamper the development of applications. In this work we reveal new insight into the connections between the performance of the graphene devices as environmental sensors and the microscopic details of the interactions at the sensing surface. Specifically, we monitor changes in the resistance of the chemical-vapour deposition grown graphene devices as exposed to different concentrations of ethanol. We perform thermal surface treatments after the devices are fabricated, use scanning probe microscopy to visualize their effects on the graphene sensing surface down to nanometer scale and correlate them with the measured performance of the device as an ethanol sensor. Our observations are compared to theoretical calculations of charge transfers between molecules and the graphene surface. We find that, although often overlooked, the surface cleanliness after device fabrication is responsible for the device performance and reliability. These results further our understanding of the mechanisms of sensing in graphene-based environmental sensors and pave the way to optimizing such devices, especially for their miniaturization, as with decreasing size of the active zone the potential role of contaminants will rise. |
2017 |
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 | Wu, Stephen M; Luican-Mayer, Adina; Bhattacharya, Anand Nanoscale measurement of Nernst effect in two-dimensional charge density wave material 1T-TaS2 Journal Article Applied Physics Letters, 111 (22), pp. 223109, 2017. @article{wu2017nanoscale, title = {Nanoscale measurement of Nernst effect in two-dimensional charge density wave material 1T-TaS2}, author = {Stephen M Wu and Adina Luican-Mayer and Anand Bhattacharya}, url = {https://doi.org/10.1063/1.5004804}, doi = {10.1063/1.5004804}, year = {2017}, date = {2017-01-01}, journal = {Applied Physics Letters}, volume = {111}, number = {22}, pages = {223109}, publisher = {AIP Publishing LLC}, abstract = {Advances in nanoscale material characterization on two-dimensional van der Waals layered materials primarily involve their optical and electronic properties. The thermal properties of these materials are harder to access due to the difficulty of thermal measurements at the nanoscale. In this work, we create a nanoscale magnetothermal device platform to access the basic out-of-plane magnetothermal transport properties of ultrathin van der Waals materials. Specifically, the Nernst effect in the charge density wave transition metal dichalcogenide 1T-TaS2 is examined on nano-thin flakes in a patterned device structure. It is revealed that near the commensurate charge density wave (CCDW) to nearly commensurate charge density wave (NCCDW) phase transition, the polarity of the Nernst effect changes. Since the Nernst effect is especially sensitive to changes in the Fermi surface, this suggests that large changes are occurring in the out-of-plane electronic structure of 1T-TaS2, which are otherwise unresolved in just in-plane electronic transport measurements. This may signal a coherent evolution of out-of-plane stacking in the CCDW → NCCDW transition. All authors acknowledge support of the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division. The use of facilities at the Center for Nanoscale Materials was supported by the U.S. DOE, BES under Contract No. DE-AC02-06CH11357.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Advances in nanoscale material characterization on two-dimensional van der Waals layered materials primarily involve their optical and electronic properties. The thermal properties of these materials are harder to access due to the difficulty of thermal measurements at the nanoscale. In this work, we create a nanoscale magnetothermal device platform to access the basic out-of-plane magnetothermal transport properties of ultrathin van der Waals materials. Specifically, the Nernst effect in the charge density wave transition metal dichalcogenide 1T-TaS2 is examined on nano-thin flakes in a patterned device structure. It is revealed that near the commensurate charge density wave (CCDW) to nearly commensurate charge density wave (NCCDW) phase transition, the polarity of the Nernst effect changes. Since the Nernst effect is especially sensitive to changes in the Fermi surface, this suggests that large changes are occurring in the out-of-plane electronic structure of 1T-TaS2, which are otherwise unresolved in just in-plane electronic transport measurements. This may signal a coherent evolution of out-of-plane stacking in the CCDW → NCCDW transition. All authors acknowledge support of the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division. The use of facilities at the Center for Nanoscale Materials was supported by the U.S. DOE, BES under Contract No. DE-AC02-06CH11357. |
 | Luican-Mayer, Adina; Li, Guohong; Andrei, Eva Y Atomic scale characterization of mismatched graphene layers Journal Article Journal of Electron Spectroscopy and Related Phenomena, 219 , pp. 92–98, 2017. @article{luican2017atomic, title = {Atomic scale characterization of mismatched graphene layers}, author = {Adina Luican-Mayer and Guohong Li and Eva Y Andrei}, url = {http://www.sciencedirect.com/science/article/pii/S0368204817300269}, doi = {https://doi.org/10.1016/j.elspec.2017.01.005}, year = {2017}, date = {2017-01-01}, journal = {Journal of Electron Spectroscopy and Related Phenomena}, volume = {219}, pages = {92--98}, publisher = {Elsevier}, abstract = {In the bourgeoning field of two dimensional layered materials and their atomically thin counterparts, it has been established that the electronic coupling between the layers of the material plays a key role in determining its properties [1,2]. We are just beginning to understand how each material is unique in that respect while working our way up to building new materials with functionalities enabled by interlayer interactions. In this review, we will focus on a system that despite its apparent simplicity possesses a wealth of intriguing physics: layers of graphene with various degree of coupling. The situations discussed here are graphene layers vertically twisted with respect with each other, weakly decoupled electronically and laterally twisted forming grain boundaries. We emphasize experiments that atomically resolve the electronic properties.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In the bourgeoning field of two dimensional layered materials and their atomically thin counterparts, it has been established that the electronic coupling between the layers of the material plays a key role in determining its properties [1,2]. We are just beginning to understand how each material is unique in that respect while working our way up to building new materials with functionalities enabled by interlayer interactions. In this review, we will focus on a system that despite its apparent simplicity possesses a wealth of intriguing physics: layers of graphene with various degree of coupling. The situations discussed here are graphene layers vertically twisted with respect with each other, weakly decoupled electronically and laterally twisted forming grain boundaries. We emphasize experiments that atomically resolve the electronic properties. |
2016 |
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 | Luican-Mayer, Adina; Barrios-Vargas, Jose E; Falkenberg, Jesper Toft; `e, Gabriel Aut; Cummings, Aron W; Soriano, David; Li, Guohong; Brandbyge, Mads; Yazyev, Oleg V; Roche, Stephan; others, Localized electronic states at grain boundaries on the surface of graphene and graphite Journal Article 2D Materials, 3 (3), pp. 031005, 2016. @article{luican2016localized, title = {Localized electronic states at grain boundaries on the surface of graphene and graphite}, author = {Adina Luican-Mayer and Jose E Barrios-Vargas and Jesper Toft Falkenberg and Gabriel Aut{`e}s and Aron W Cummings and David Soriano and Guohong Li and Mads Brandbyge and Oleg V Yazyev and Stephan Roche and others}, url = {https://doi.org/10.1088%2F2053-1583%2F3%2F3%2F031005}, doi = {10.1088/2053-1583/3/3/031005}, year = {2016}, date = {2016-01-01}, journal = {2D Materials}, volume = {3}, number = {3}, pages = {031005}, publisher = {IOP Publishing}, abstract = {Recent advances in large-scale synthesis of graphene and other 2D materials have underscored the importance of local defects such as dislocations and grain boundaries (GBs), and especially their tendency to alter the electronic properties of the material. Understanding how the polycrystalline morphology affects the electronic properties is crucial for the development of applications such as flexible electronics, energy harvesting devices or sensors. We here report on atomic scale characterization of several GBs and on the structural-dependence of the localized electronic states in their vicinity. Using low temperature scanning tunneling microscopy and spectroscopy, together with tight binding and ab initio numerical simulations we explore GBs on the surface of graphite and elucidate the interconnection between the local density of states and their atomic structure. We show that the electronic fingerprints of these GBs consist of pronounced resonances which, depending on the relative orientation of the adjacent crystallites, appear either on the electron side of the spectrum or as an electron-hole symmetric doublet close to the charge neutrality point. These two types of spectral features will impact very differently the transport properties allowing, in the asymmetric case to introduce transport anisotropy which could be utilized to design novel growth and fabrication strategies to control device performance.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Recent advances in large-scale synthesis of graphene and other 2D materials have underscored the importance of local defects such as dislocations and grain boundaries (GBs), and especially their tendency to alter the electronic properties of the material. Understanding how the polycrystalline morphology affects the electronic properties is crucial for the development of applications such as flexible electronics, energy harvesting devices or sensors. We here report on atomic scale characterization of several GBs and on the structural-dependence of the localized electronic states in their vicinity. Using low temperature scanning tunneling microscopy and spectroscopy, together with tight binding and ab initio numerical simulations we explore GBs on the surface of graphite and elucidate the interconnection between the local density of states and their atomic structure. We show that the electronic fingerprints of these GBs consist of pronounced resonances which, depending on the relative orientation of the adjacent crystallites, appear either on the electron side of the spectrum or as an electron-hole symmetric doublet close to the charge neutrality point. These two types of spectral features will impact very differently the transport properties allowing, in the asymmetric case to introduce transport anisotropy which could be utilized to design novel growth and fabrication strategies to control device performance. |
 | Lu, Chih-Pin; Rodriguez-Vega, Martin; Li, Guohong; Luican-Mayer, Adina; Watanabe, Kenji; Taniguchi, Takashi; Rossi, Enrico; Andrei, Eva Y Local, global, and nonlinear screening in twisted double-layer graphene Journal Article Proceedings of the National Academy of Sciences, 113 (24), pp. 6623–6628, 2016. @article{lu2016local, title = {Local, global, and nonlinear screening in twisted double-layer graphene}, author = {Chih-Pin Lu and Martin Rodriguez-Vega and Guohong Li and Adina Luican-Mayer and Kenji Watanabe and Takashi Taniguchi and Enrico Rossi and Eva Y Andrei}, url = {https://www.pnas.org/content/113/24/6623/tab-article-info}, doi = {https://doi.org/10.1073/pnas.1606278113}, year = {2016}, date = {2016-01-01}, journal = {Proceedings of the National Academy of Sciences}, volume = {113}, number = {24}, pages = {6623--6628}, publisher = {National Acad Sciences}, abstract = {One-atom-thick crystalline layers and their vertical heterostructures carry the promise of designer electronic materials that are unattainable by standard growth techniques. To realize their potential it is necessary to isolate them from environmental disturbances, in particular those introduced by the substrate. However, finding and characterizing suitable substrates, and minimizing the random potential fluctuations they introduce, has been a persistent challenge in this emerging field. Here we show that Landau-level (LL) spectroscopy offers the unique capability to quantify both the reduction of the quasiparticles’ lifetime and the long-range inhomogeneity due to random potential fluctuations. Harnessing this technique together with direct scanning tunneling microscopy and numerical simulations we demonstrate that the insertion of a graphene buffer layer with a large twist angle is a very effective method to shield a 2D system from substrate interference that has the additional desirable property of preserving the electronic structure of the system under study. We further show that owing to its remarkable nonlinear screening capability a single graphene buffer layer provides better shielding than either increasing the distance to the substrate or doubling the carrier density and reduces the amplitude of the potential fluctuations in graphene to values even lower than the ones in AB-stacked bilayer graphene.}, keywords = {}, pubstate = {published}, tppubtype = {article} } One-atom-thick crystalline layers and their vertical heterostructures carry the promise of designer electronic materials that are unattainable by standard growth techniques. To realize their potential it is necessary to isolate them from environmental disturbances, in particular those introduced by the substrate. However, finding and characterizing suitable substrates, and minimizing the random potential fluctuations they introduce, has been a persistent challenge in this emerging field. Here we show that Landau-level (LL) spectroscopy offers the unique capability to quantify both the reduction of the quasiparticles’ lifetime and the long-range inhomogeneity due to random potential fluctuations. Harnessing this technique together with direct scanning tunneling microscopy and numerical simulations we demonstrate that the insertion of a graphene buffer layer with a large twist angle is a very effective method to shield a 2D system from substrate interference that has the additional desirable property of preserving the electronic structure of the system under study. We further show that owing to its remarkable nonlinear screening capability a single graphene buffer layer provides better shielding than either increasing the distance to the substrate or doubling the carrier density and reduces the amplitude of the potential fluctuations in graphene to values even lower than the ones in AB-stacked bilayer graphene. |
2015 |
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Thoutam, LR; Wang, YL; Xiao, ZL; Das, S; Luican-Mayer, A; Divan, R; Crabtree, GW; Kwok, WK Temperature-dependent three-dimensional anisotropy of the magnetoresistance in WTe 2 Journal Article Physical review letters, 115 (4), pp. 046602, 2015. @article{thoutam2015temperature, title = {Temperature-dependent three-dimensional anisotropy of the magnetoresistance in WTe 2}, author = {LR Thoutam and YL Wang and ZL Xiao and S Das and A Luican-Mayer and R Divan and GW Crabtree and WK Kwok}, url = {https://link.aps.org/doi/10.1103/PhysRevLett.115.046602}, doi = {10.1103/PhysRevLett.115.046602}, year = {2015}, date = {2015-01-01}, journal = {Physical review letters}, volume = {115}, number = {4}, pages = {046602}, publisher = {APS}, abstract = {Extremely large magnetoresistance (XMR) was recently discovered in WTe2, triggering extensive research on this material regarding the XMR origin. Since WTe2 is a layered compound with metal layers sandwiched between adjacent insulating chalcogenide layers, this material has been considered to be electronically two-dimensional (2D). Here we report two new findings on WTe2: (1) WTe2 is electronically 3D with a mass anisotropy as low as 2, as revealed by the 3D scaling behavior of the resistance R(H,θ)=R(ϵθH) with ϵθ=(cos2θ+γ−2sin2θ)1/2, θ being the magnetic field angle with respect to the c axis of the crystal and γ being the mass anisotropy and (2) the mass anisotropy γ varies with temperature and follows the magnetoresistance behavior of the Fermi liquid state. Our results not only provide a general scaling approach for the anisotropic magnetoresistance but also are crucial for correctly understanding the electronic properties of WTe2, including the origin of the remarkable “turn-on” behavior in the resistance versus temperature curve, which has been widely observed in many materials and assumed to be a metal-insulator transition.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Extremely large magnetoresistance (XMR) was recently discovered in WTe2, triggering extensive research on this material regarding the XMR origin. Since WTe2 is a layered compound with metal layers sandwiched between adjacent insulating chalcogenide layers, this material has been considered to be electronically two-dimensional (2D). Here we report two new findings on WTe2: (1) WTe2 is electronically 3D with a mass anisotropy as low as 2, as revealed by the 3D scaling behavior of the resistance R(H,θ)=R(ϵθH) with ϵθ=(cos2θ+γ−2sin2θ)1/2, θ being the magnetic field angle with respect to the c axis of the crystal and γ being the mass anisotropy and (2) the mass anisotropy γ varies with temperature and follows the magnetoresistance behavior of the Fermi liquid state. Our results not only provide a general scaling approach for the anisotropic magnetoresistance but also are crucial for correctly understanding the electronic properties of WTe2, including the origin of the remarkable “turn-on” behavior in the resistance versus temperature curve, which has been widely observed in many materials and assumed to be a metal-insulator transition. | |
Wang, YL; Thoutam, LR; Xiao, ZL; Hu, J; Das, S; Mao, ZQ; Wei, J; Divan, R; Luican-Mayer, A; Crabtree, GW; others, Origin of the turn-on temperature behavior in WTe 2 Journal Article Physical Review B, 92 (18), pp. 180402, 2015. @article{wang2015origin, title = {Origin of the turn-on temperature behavior in WTe 2}, author = {YL Wang and LR Thoutam and ZL Xiao and J Hu and S Das and ZQ Mao and J Wei and R Divan and A Luican-Mayer and GW Crabtree and others}, url = {https://link.aps.org/doi/10.1103/PhysRevB.92.180402}, doi = {10.1103/PhysRevB.92.180402}, year = {2015}, date = {2015-01-01}, journal = {Physical Review B}, volume = {92}, number = {18}, pages = {180402}, publisher = {APS}, abstract = {A hallmark of materials with extremely large magnetoresistance (XMR) is the transformative turn-on temperature behavior: when the applied magnetic field H is above certain value, the resistivity versus temperature ρ(T) curve shows a minimum at a field dependent temperature T∗, which has been interpreted as a magnetic-field-driven metal-insulator transition or attributed to an electronic structure change. Here, we demonstrate that ρ(T) curves with turn-on behavior in the newly discovered XMR material WTe2 can be scaled as MR∼(H/ρ0)m with m≈2 and ρ0 being the resistivity at zero field. We obtained experimentally and also derived from the observed scaling the magnetic field dependence of the turn-on temperature T∗∼(H−Hc)ν with ν≈1/2, which was earlier used as evidence for a predicted metal-insulator transition. The scaling also leads to a simple quantitative expression for the resistivity ρ∗≈2ρ0 at the onset of the XMR behavior, which fits the data remarkably well. These results exclude the possible existence of a magnetic-field-driven metal-insulator transition or significant contribution of an electronic structure change to the low-temperature XMR in WTe2. This work resolves the origin of the turn-on behavior observed in several XMR materials and also provides a general route for a quantitative understanding of the temperature dependence of MR in both XMR and non-XMR materials.}, keywords = {}, pubstate = {published}, tppubtype = {article} } A hallmark of materials with extremely large magnetoresistance (XMR) is the transformative turn-on temperature behavior: when the applied magnetic field H is above certain value, the resistivity versus temperature ρ(T) curve shows a minimum at a field dependent temperature T∗, which has been interpreted as a magnetic-field-driven metal-insulator transition or attributed to an electronic structure change. Here, we demonstrate that ρ(T) curves with turn-on behavior in the newly discovered XMR material WTe2 can be scaled as MR∼(H/ρ0)m with m≈2 and ρ0 being the resistivity at zero field. We obtained experimentally and also derived from the observed scaling the magnetic field dependence of the turn-on temperature T∗∼(H−Hc)ν with ν≈1/2, which was earlier used as evidence for a predicted metal-insulator transition. The scaling also leads to a simple quantitative expression for the resistivity ρ∗≈2ρ0 at the onset of the XMR behavior, which fits the data remarkably well. These results exclude the possible existence of a magnetic-field-driven metal-insulator transition or significant contribution of an electronic structure change to the low-temperature XMR in WTe2. This work resolves the origin of the turn-on behavior observed in several XMR materials and also provides a general route for a quantitative understanding of the temperature dependence of MR in both XMR and non-XMR materials. | |
2013 |
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Luican-Mayer, Adina; Kharitonov, Maxim; Li, Guohong; Lu, ChihPin; Skachko, Ivan; Goncalves, Alem-Mar; Andrei, Eva Y Visualizing the influence of an isolated Coulomb impurity on the Landau level spectrum in graphene using scanning tunneling microscopy Journal Article APS, 2013 , pp. U8–003, 2013. @article{luican2013visualizing, title = {Visualizing the influence of an isolated Coulomb impurity on the Landau level spectrum in graphene using scanning tunneling microscopy}, author = {Adina Luican-Mayer and Maxim Kharitonov and Guohong Li and ChihPin Lu and Ivan Skachko and Alem-Mar Goncalves and Eva Y Andrei}, url = {https://ui.adsabs.harvard.edu/abs/2013APS..MAR.U8003L/abstract}, year = {2013}, date = {2013-01-01}, journal = {APS}, volume = {2013}, pages = {U8--003}, abstract = {Charged impurities play a crucial role in determining the electronic properties of graphene. We report on experiments that elucidate the effect of an isolated charged impurity on the electronic spectrum of graphene in a magnetic field. Using scanning tunneling microscopy and gated graphene devices, we follow the evolution of quantized Landau levels with carrier density and find that the apparent strength of the impurity is controlled by the partial filling of the Landau levels. At low filling the impurity is cloaked and becomes essentially invisible. The cloaking effect diminishes with filling until, for fully occupied Landau levels, the impurity reaches its maximum strength causing a significant perturbation in the local density of states. In this regime we report the first observation of Landau level splitting due to lifting of the orbital degeneracy.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Charged impurities play a crucial role in determining the electronic properties of graphene. We report on experiments that elucidate the effect of an isolated charged impurity on the electronic spectrum of graphene in a magnetic field. Using scanning tunneling microscopy and gated graphene devices, we follow the evolution of quantized Landau levels with carrier density and find that the apparent strength of the impurity is controlled by the partial filling of the Landau levels. At low filling the impurity is cloaked and becomes essentially invisible. The cloaking effect diminishes with filling until, for fully occupied Landau levels, the impurity reaches its maximum strength causing a significant perturbation in the local density of states. In this regime we report the first observation of Landau level splitting due to lifting of the orbital degeneracy. | |
Li, Guohong; Luican-Mayer, Adina; Abanin, Dmitry; Levitov, Leonid; Andrei, Eva Y Evolution of Landau levels into edge states in graphene Journal Article Nature communications, 4 (1), pp. 1–7, 2013. @article{li2013evolution, title = {Evolution of Landau levels into edge states in graphene}, author = {Guohong Li and Adina Luican-Mayer and Dmitry Abanin and Leonid Levitov and Eva Y Andrei}, url = {https://www.nature.com/articles/ncomms2767}, doi = {https://doi.org/10.1038/ncomms2767}, year = {2013}, date = {2013-01-01}, journal = {Nature communications}, volume = {4}, number = {1}, pages = {1--7}, publisher = {Nature Publishing Group}, abstract = {Two-dimensional electron systems in the presence of a magnetic field support topologically ordered states, in which the coexistence of an insulating bulk with conducting one-dimensional chiral edge states gives rise to the quantum Hall effect. For systems confined by sharp boundaries, theory predicts a unique edge-bulk correspondence, which is central to proposals of quantum Hall-based topological qubits. However, in conventional semiconductor-based two-dimensional electron systems, these elegant concepts are difficult to realize, because edge-state reconstruction due to soft boundaries destroys the edge-bulk correspondence. Here we use scanning tunnelling microscopy and spectroscopy to follow the spatial evolution of electronic (Landau) levels towards an edge of graphene supported above a graphite substrate. We observe no edge-state reconstruction, in agreement with calculations based on an atomically sharp boundary. Our results single out graphene as a system where the edge structure can be controlled and the edge-bulk correspondence is preserved.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Two-dimensional electron systems in the presence of a magnetic field support topologically ordered states, in which the coexistence of an insulating bulk with conducting one-dimensional chiral edge states gives rise to the quantum Hall effect. For systems confined by sharp boundaries, theory predicts a unique edge-bulk correspondence, which is central to proposals of quantum Hall-based topological qubits. However, in conventional semiconductor-based two-dimensional electron systems, these elegant concepts are difficult to realize, because edge-state reconstruction due to soft boundaries destroys the edge-bulk correspondence. Here we use scanning tunnelling microscopy and spectroscopy to follow the spatial evolution of electronic (Landau) levels towards an edge of graphene supported above a graphite substrate. We observe no edge-state reconstruction, in agreement with calculations based on an atomically sharp boundary. Our results single out graphene as a system where the edge structure can be controlled and the edge-bulk correspondence is preserved. | |
2012 |
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Luican-Mayer, Adina Scanning tunneling microscopy and spectroscopy studies of graphene Book Citeseer, 2012. @book{luican2012scanning, title = {Scanning tunneling microscopy and spectroscopy studies of graphene}, author = {Adina Luican-Mayer}, url = {https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.914.1608&rep=rep1&type=pdf}, year = {2012}, date = {2012-01-01}, publisher = {Citeseer}, abstract = {In the two-dimensional (2D) lattice of graphene, consisting of carbon atoms arranged in a honeycomb lattice, the charge carriers are described by a Dirac-Weyl Hamiltonian. Seeking to understand their unique nature, this thesis presents results of scanning tunneling microscopy (STM) and spectroscopy (STS) experiments at low temperatures and in magnetic field. These techniques give access, down to atomic scales, to structural information as well as to the electronic properties of graphene. The main findings include the observation of quantized Landau levels (LL) in the presence of magnetic field, their dependence on carrier density and effects of charged impurities and other disorder on the LL spectrum. Twisting graphene layers away from the equilibrium Bernal stacking leads to the formation of Moir´e patterns that significantly alter the electronic properties of graphene stacks. The second part of the thesis discusses the effects of such rotations on the electronic properties as a function of twist angle.}, keywords = {}, pubstate = {published}, tppubtype = {book} } In the two-dimensional (2D) lattice of graphene, consisting of carbon atoms arranged in a honeycomb lattice, the charge carriers are described by a Dirac-Weyl Hamiltonian. Seeking to understand their unique nature, this thesis presents results of scanning tunneling microscopy (STM) and spectroscopy (STS) experiments at low temperatures and in magnetic field. These techniques give access, down to atomic scales, to structural information as well as to the electronic properties of graphene. The main findings include the observation of quantized Landau levels (LL) in the presence of magnetic field, their dependence on carrier density and effects of charged impurities and other disorder on the LL spectrum. Twisting graphene layers away from the equilibrium Bernal stacking leads to the formation of Moir´e patterns that significantly alter the electronic properties of graphene stacks. The second part of the thesis discusses the effects of such rotations on the electronic properties as a function of twist angle. | |
2011 |
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Luican, A; Li, Guohong; Reina, A; Kong, J; Nair, RR; Novoselov, Konstantin S; Geim, Andre K; Andrei, EY Single-layer behavior and its breakdown in twisted graphene layers Journal Article Physical review letters, 106 (12), pp. 126802, 2011. @article{luican2011single, title = {Single-layer behavior and its breakdown in twisted graphene layers}, author = {A Luican and Guohong Li and A Reina and J Kong and RR Nair and Konstantin S Novoselov and Andre K Geim and EY Andrei}, url = {https://link.aps.org/doi/10.1103/PhysRevLett.106.126802}, doi = {10.1103/PhysRevLett.106.126802}, year = {2011}, date = {2011-01-01}, journal = {Physical review letters}, volume = {106}, number = {12}, pages = {126802}, publisher = {APS}, abstract = {We report high magnetic field scanning tunneling microscopy and Landau level spectroscopy of twisted graphene layers grown by chemical vapor deposition. For twist angles exceeding ∼3° the low energy carriers exhibit Landau level spectra characteristic of massless Dirac fermions. Above 20° the layers effectively decouple and the electronic properties are indistinguishable from those in single-layer graphene, while for smaller angles we observe a slowdown of the carrier velocity which is strongly angle dependent. At the smallest angles the spectra are dominated by twist-induced van Hove singularities and the Dirac fermions eventually become localized. An unexpected electron-hole asymmetry is observed which is substantially larger than the asymmetry in either single or untwisted bilayer graphene.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We report high magnetic field scanning tunneling microscopy and Landau level spectroscopy of twisted graphene layers grown by chemical vapor deposition. For twist angles exceeding ∼3° the low energy carriers exhibit Landau level spectra characteristic of massless Dirac fermions. Above 20° the layers effectively decouple and the electronic properties are indistinguishable from those in single-layer graphene, while for smaller angles we observe a slowdown of the carrier velocity which is strongly angle dependent. At the smallest angles the spectra are dominated by twist-induced van Hove singularities and the Dirac fermions eventually become localized. An unexpected electron-hole asymmetry is observed which is substantially larger than the asymmetry in either single or untwisted bilayer graphene. | |
Luican, Adina; Li, Guohong; Andrei, Eva Y Quantized Landau level spectrum and its density dependence in graphene Journal Article Physical Review B, 83 (4), pp. 041405, 2011. @article{luican2011quantized, title = {Quantized Landau level spectrum and its density dependence in graphene}, author = {Adina Luican and Guohong Li and Eva Y Andrei}, url = {https://link.aps.org/doi/10.1103/PhysRevB.83.041405}, doi = {10.1103/PhysRevB.83.041405}, year = {2011}, date = {2011-01-01}, journal = {Physical Review B}, volume = {83}, number = {4}, pages = {041405}, publisher = {APS}, abstract = {Scanning tunneling microscopy and spectroscopy in a magnetic field was used to study Landau quantization in graphene and its dependence on charge carrier density. Measurements were carried out on exfoliated graphene samples deposited on a chlorinated SiO2 thermal oxide, which allowed for the observation of the Landau level sequences characteristic of single-layer graphene while tuning the density through the Si backgate. Upon changing the carrier density, we find abrupt jumps in the Fermi level after each Landau level is filled. Moreover, at low doping levels, a marked increase in the Fermi velocity is observed, which is consistent with the logarithmic divergence expected due to the onset of many-body effects close to the Dirac point.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Scanning tunneling microscopy and spectroscopy in a magnetic field was used to study Landau quantization in graphene and its dependence on charge carrier density. Measurements were carried out on exfoliated graphene samples deposited on a chlorinated SiO2 thermal oxide, which allowed for the observation of the Landau level sequences characteristic of single-layer graphene while tuning the density through the Si backgate. Upon changing the carrier density, we find abrupt jumps in the Fermi level after each Landau level is filled. Moreover, at low doping levels, a marked increase in the Fermi velocity is observed, which is consistent with the logarithmic divergence expected due to the onset of many-body effects close to the Dirac point. | |
Li, Guohong; Luican, Adina; Andrei, Eva Y Self-navigation of a scanning tunneling microscope tip toward a micron-sized graphene sample Journal Article Review of Scientific Instruments, 82 (7), pp. 073701, 2011. @article{li2011self, title = {Self-navigation of a scanning tunneling microscope tip toward a micron-sized graphene sample}, author = {Guohong Li and Adina Luican and Eva Y Andrei}, url = {https://aip.scitation.org/doi/10.1063/1.3605664}, doi = {https://doi.org/10.1063/1.3605664}, year = {2011}, date = {2011-01-01}, journal = {Review of Scientific Instruments}, volume = {82}, number = {7}, pages = {073701}, publisher = {American Institute of Physics}, abstract = {We demonstrate a simple capacitance-based method to quickly and efficiently locate micron-sized conductive samples, such as graphene flakes, on insulating substrates in a scanning tunneling microscope (STM). By using edge recognition, the method is designed to locate and to identify small features when the STM tip is far above the surface, allowing for crash-free search and navigation. The method can be implemented in any STM environment, even at low temperatures and in strong magnetic field, with minimal or no hardware modifications. }, keywords = {}, pubstate = {published}, tppubtype = {article} } We demonstrate a simple capacitance-based method to quickly and efficiently locate micron-sized conductive samples, such as graphene flakes, on insulating substrates in a scanning tunneling microscope (STM). By using edge recognition, the method is designed to locate and to identify small features when the STM tip is far above the surface, allowing for crash-free search and navigation. The method can be implemented in any STM environment, even at low temperatures and in strong magnetic field, with minimal or no hardware modifications. | |
2010 |
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Li, Guohong; Luican, A; dos Santos, Lopes J M B; Neto, Castro A H; Reina, A; Kong, J; Andrei, E Y Observation of Van Hove singularities in twisted graphene layers Journal Article Nature Physics, 6 (2), pp. 109–113, 2010, ISSN: 1745-2481. @article{li_observation_2010, title = {Observation of Van Hove singularities in twisted graphene layers}, author = {Guohong Li and A Luican and J M B Lopes dos Santos and A H Castro Neto and A Reina and J Kong and E Y Andrei}, url = {https://www.nature.com/articles/nphys1463}, doi = {10.1038/nphys1463}, issn = {1745-2481}, year = {2010}, date = {2010-01-01}, urldate = {2020-06-17}, journal = {Nature Physics}, volume = {6}, number = {2}, pages = {109--113}, abstract = {When a Van Hove singularity exists near the Fermi energy of a solid’s density of states, it can cause a variety of exotic phenomena to emerge. Scanning tunnelling microscope measurements indicate that when graphite’s graphene sheets are rotated out of their usual alignment, it can generate low-energy Van Hove singularities for which the position is controlled by the angle of rotation.}, keywords = {}, pubstate = {published}, tppubtype = {article} } When a Van Hove singularity exists near the Fermi energy of a solid’s density of states, it can cause a variety of exotic phenomena to emerge. Scanning tunnelling microscope measurements indicate that when graphite’s graphene sheets are rotated out of their usual alignment, it can generate low-energy Van Hove singularities for which the position is controlled by the angle of rotation. | |
2009 |
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Du, Xu; Skachko, Ivan; Duerr, Fabian; Luican, Adina; Andrei, Eva Y Fractional quantum Hall effect and insulating phase of Dirac electrons in graphene Journal Article Nature, 462 (7270), pp. 192-195, 2009, ISSN: 1476-4687. @article{Du2009b, title = {Fractional quantum Hall effect and insulating phase of Dirac electrons in graphene}, author = {Xu Du and Ivan Skachko and Fabian Duerr and Adina Luican and Eva Y Andrei}, url = {https://doi.org/10.1038/nature08522}, doi = {10.1038/nature08522}, issn = {1476-4687}, year = {2009}, date = {2009-11-01}, journal = {Nature}, volume = {462}, number = {7270}, pages = {192-195}, abstract = {The fractional quantum Hall effect is a quintessential manifestation of the collective behaviour associated with strongly interacting charge carriers confined to two dimensions and subject to a strong magnetic field. It is predicted that the charge carriers present in graphene --- an atomic layer of carbon that can be seen as the 'perfect' two-dimensional system --- are subject to strong interactions. Nevertheless, the phenomenon had eluded experimental observation until now: in this issue two groups report fractional quantum Hall effect in suspended sheets of graphene, probed in a two-terminal measurement setup. The researchers also observe a magnetic-field-induced insulating state at low carrier density, which competes with the quantum Hall effect and limits its observation to the highest-quality samples only. These results pave the way for the study of the rich collective behaviour of Dirac fermions in graphene.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The fractional quantum Hall effect is a quintessential manifestation of the collective behaviour associated with strongly interacting charge carriers confined to two dimensions and subject to a strong magnetic field. It is predicted that the charge carriers present in graphene --- an atomic layer of carbon that can be seen as the 'perfect' two-dimensional system --- are subject to strong interactions. Nevertheless, the phenomenon had eluded experimental observation until now: in this issue two groups report fractional quantum Hall effect in suspended sheets of graphene, probed in a two-terminal measurement setup. The researchers also observe a magnetic-field-induced insulating state at low carrier density, which competes with the quantum Hall effect and limits its observation to the highest-quality samples only. These results pave the way for the study of the rich collective behaviour of Dirac fermions in graphene. | |
Luican, Adina; Li, Guohong; Andrei, Eva Y Scanning tunneling microscopy and spectroscopy of graphene layers on graphite Journal Article Solid State Communications, 149 (27-28), pp. 1151–1156, 2009. @article{luican2009scanning, title = {Scanning tunneling microscopy and spectroscopy of graphene layers on graphite}, author = {Adina Luican and Guohong Li and Eva Y Andrei}, url = {https://www.sciencedirect.com/science/article/abs/pii/S0038109809001446}, doi = {https://doi.org/10.1016/j.ssc.2009.02.059}, year = {2009}, date = {2009-01-01}, journal = {Solid State Communications}, volume = {149}, number = {27-28}, pages = {1151--1156}, publisher = {Elsevier}, abstract = {We report low temperature scanning tunneling microscopy and spectroscopy on graphene flakes supported on a graphite substrate. The experiments demonstrate that graphite is exceptionally well suited as a substrate for graphene because it offers support without disturbing the intrinsic properties of the charge carriers. The degree of coupling of a graphene flake to the substrate was recognized and characterized from the appearance of an anomalous Landau level sequence in the presence of a perpendicular magnetic field. By following the evolution of the Landau level spectra along the surface, we identified graphene flakes that are decoupled or very weakly coupled to the substrate. From the Landau level sequence in this flake, we extract the local Fermi velocity and energy of the Dirac point and find extremely weak spatial variation of these quantities confirming the high quality and non invasive nature of the graphite substrate.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We report low temperature scanning tunneling microscopy and spectroscopy on graphene flakes supported on a graphite substrate. The experiments demonstrate that graphite is exceptionally well suited as a substrate for graphene because it offers support without disturbing the intrinsic properties of the charge carriers. The degree of coupling of a graphene flake to the substrate was recognized and characterized from the appearance of an anomalous Landau level sequence in the presence of a perpendicular magnetic field. By following the evolution of the Landau level spectra along the surface, we identified graphene flakes that are decoupled or very weakly coupled to the substrate. From the Landau level sequence in this flake, we extract the local Fermi velocity and energy of the Dirac point and find extremely weak spatial variation of these quantities confirming the high quality and non invasive nature of the graphite substrate. | |
Li, Guohong; Luican, Adina; Andrei, Eva Y Electronic states on the surface of graphite Journal Article Physica B: Condensed Matter, 404 (18), pp. 2673–2677, 2009. @article{li2009electronic, title = {Electronic states on the surface of graphite}, author = {Guohong Li and Adina Luican and Eva Y Andrei}, url = {https://www.sciencedirect.com/science/article/abs/pii/S0921452609004165}, doi = {https://doi.org/10.1016/j.physb.2009.06.071}, year = {2009}, date = {2009-01-01}, journal = {Physica B: Condensed Matter}, volume = {404}, number = {18}, pages = {2673--2677}, publisher = {Elsevier}, abstract = {Graphite consists of graphene layers in an AB (Bernal) stacking arrangement. The introduction of defects can reduce the coupling between the top graphene layers and the bulk crystal producing new electronic states that reflect the degree of coupling. We employ low temperature high magnetic field scanning tunneling microscopy (STM) and spectroscopy (STS) to access these states and study their evolution with the degree of coupling. STS in magnetic field directly probes the dimensionality of electronic states. Thus two-dimensional states produce a discrete series of Landau levels while three-dimensional states form Landau bands providing a clear distinction between completely decoupled top layers and ones that are coupled to the substrate. We show that the completely decoupled layers are characterized by a single sequence of Landau levels with square-root dependence on field and level index indicative of massless Dirac fermions. In contrast weakly coupled bilayers produce special sequences reflecting the degree of coupling, and multilayers produce sequences reflecting the coexistence of massless and massive Dirac fermions. In addition we show that the graphite surface is soft and that an STM tip can be quite invasive when brought too close to the surface and that there is a characteristic tip–sample distance beyond which the effect of sample–tip interaction is negligible.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Graphite consists of graphene layers in an AB (Bernal) stacking arrangement. The introduction of defects can reduce the coupling between the top graphene layers and the bulk crystal producing new electronic states that reflect the degree of coupling. We employ low temperature high magnetic field scanning tunneling microscopy (STM) and spectroscopy (STS) to access these states and study their evolution with the degree of coupling. STS in magnetic field directly probes the dimensionality of electronic states. Thus two-dimensional states produce a discrete series of Landau levels while three-dimensional states form Landau bands providing a clear distinction between completely decoupled top layers and ones that are coupled to the substrate. We show that the completely decoupled layers are characterized by a single sequence of Landau levels with square-root dependence on field and level index indicative of massless Dirac fermions. In contrast weakly coupled bilayers produce special sequences reflecting the degree of coupling, and multilayers produce sequences reflecting the coexistence of massless and massive Dirac fermions. In addition we show that the graphite surface is soft and that an STM tip can be quite invasive when brought too close to the surface and that there is a characteristic tip–sample distance beyond which the effect of sample–tip interaction is negligible. | |
Skachko, I; Du, X; Duerr, F; Luican, A; Abanin, DA; Levitov, LS; Andrei, EY Integer and fractional quantum hall effect in two-terminal measurements on suspended graphene Journal Article arXiv preprint arXiv:0910.2518, 2009. @article{skachko2009integer, title = {Integer and fractional quantum hall effect in two-terminal measurements on suspended graphene}, author = {I Skachko and X Du and F Duerr and A Luican and DA Abanin and LS Levitov and EY Andrei}, url = {https://arxiv.org/abs/0910.2518}, doi = {10.1098/rsta.2010.0226}, year = {2009}, date = {2009-01-01}, journal = {arXiv preprint arXiv:0910.2518}, abstract = {We report the observation of the quantized Hall effect in suspended graphene probed with a two-terminal lead geometry. The failure of earlier Hall-bar measurements is discussed and attributed to the placement of voltage probes in mesoscopic samples. New quantized states are found at integer Landau level fillings outside the sequence 2,6,10.., as well as at a fractional filling \nu=1/3. Their presence is revealed by plateaus in the two-terminal conductance which appear in magnetic fields as low as 2 Tesla at low temperatures and persist up to 20 Kelvin in 12 Tesla. The excitation gaps, extracted from the data with the help of a theoretical model, are found to be significantly larger than in GaAs based electron systems.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We report the observation of the quantized Hall effect in suspended graphene probed with a two-terminal lead geometry. The failure of earlier Hall-bar measurements is discussed and attributed to the placement of voltage probes in mesoscopic samples. New quantized states are found at integer Landau level fillings outside the sequence 2,6,10.., as well as at a fractional filling nu=1/3. Their presence is revealed by plateaus in the two-terminal conductance which appear in magnetic fields as low as 2 Tesla at low temperatures and persist up to 20 Kelvin in 12 Tesla. The excitation gaps, extracted from the data with the help of a theoretical model, are found to be significantly larger than in GaAs based electron systems. | |
2006 |
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Temirov, R; Soubatch, S; Luican, A; Tautz, FS Free-electron-like dispersion in an organic monolayer film on a metal substrate Journal Article Nature, 444 (7117), pp. 350–353, 2006. @article{temirov2006free, title = {Free-electron-like dispersion in an organic monolayer film on a metal substrate}, author = {R Temirov and S Soubatch and A Luican and FS Tautz}, url = {https://www.nature.com/articles/nature05270}, doi = {https://doi.org/10.1038/nature05270}, year = {2006}, date = {2006-01-01}, journal = {Nature}, volume = {444}, number = {7117}, pages = {350--353}, publisher = {Nature Publishing Group}, abstract = {Thin films of molecular organic semiconductors are attracting much interest for use in electronic and optoelectronic applications. The electronic properties of these materials and their interfaces are therefore worth investigating intensively1,2,3, particularly the degree of electron delocalization that can be achieved2,4. If the delocalization is appreciable, it should be accompanied by an observable electronic band dispersion. But so far only limited experimental data on the intermolecular dispersion of electronic states in molecular materials is available5,6,7,8, and the mechanism(s) of electron delocalization in molecular materials are also not well understood. Here we report scanning tunnelling spectroscopy observations of an organic monolayer film on a silver substrate, revealing a completely delocalized two-dimensional band state that is characterized by a metal-like parabolic dispersion with an effective mass of m* = 0.47me, where me is the bare electron mass. This dispersion is far stronger than expected for the organic film alone7, and arises as a result of strong substrate-mediated coupling between the molecules within the monolayer.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Thin films of molecular organic semiconductors are attracting much interest for use in electronic and optoelectronic applications. The electronic properties of these materials and their interfaces are therefore worth investigating intensively1,2,3, particularly the degree of electron delocalization that can be achieved2,4. If the delocalization is appreciable, it should be accompanied by an observable electronic band dispersion. But so far only limited experimental data on the intermolecular dispersion of electronic states in molecular materials is available5,6,7,8, and the mechanism(s) of electron delocalization in molecular materials are also not well understood. Here we report scanning tunnelling spectroscopy observations of an organic monolayer film on a silver substrate, revealing a completely delocalized two-dimensional band state that is characterized by a metal-like parabolic dispersion with an effective mass of m* = 0.47me, where me is the bare electron mass. This dispersion is far stronger than expected for the organic film alone7, and arises as a result of strong substrate-mediated coupling between the molecules within the monolayer. | |