We report the use of an ultralow noise nano-superconducting quantum interference device (nanoSQUI... more We report the use of an ultralow noise nano-superconducting quantum interference device (nanoSQUID) to measure the hysteretic magnetization behavior of a single FePt nanobead at a temperature of around 7 K in a magnetic field of only ~10 mT. We also show that the nanobead can be accurately positioned with respect to the SQUID loop and then removed without affecting
Large area superconducting quantum interference devices (SQUIDs) are known to be ultimate sensors... more Large area superconducting quantum interference devices (SQUIDs) are known to be ultimate sensors of magnetic flux. By diminishing the size of a SQUID loop, the energy sensitivity of the device can be increased to near quantum-limited operation whilst also making it less sensitive to external magnetic field noise and improving its coupling to any nano-scale magnetic particle within the loop.
This paper deals with the modeling of sensitivity of epitaxial graphene Hall bars, from sub-micro... more This paper deals with the modeling of sensitivity of epitaxial graphene Hall bars, from sub-micrometer to micrometer size, to the stray field generated by a magnetic microbead. To demonstrate experiment feasibility, the model is first validated by comparison to measurement results, considering an ac-dc detection scheme. Then, a comprehensive numerical analysis is performed to investigate signal detriment caused by sensor material heterogeneities, saturation of bead magnetization at high fields, increment of bead distance from sensor surface, and device width increase.
This article addresses the much debated question whether the degree of hydrophobicity of single-l... more This article addresses the much debated question whether the degree of hydrophobicity of single-layer graphene (1LG) is different from that of double-layer graphene (2LG). Knowledge of the water affinity of graphene and its spatial variations is critically important as it can affect the graphene properties as well as the performance of graphene devices exposed to humidity. By employing chemical force microscopy with a probe rendered hydrophobic by functionalization with octadecyltrichlorosilane (OTS), the adhesion force between the probe and epitaxial graphene on SiC has been measured in deionized water. Owing to the hydrophobic attraction, a larger adhesion force was measured on 2LG Bernal-stacked domains of graphene surfaces, thus showing that 2LG is more hydrophobic than 1LG. Identification of 1LG and 2LG domains was achieved through Kelvin probe force microscopy and Raman spectral mapping. Approximate values of the adhesion force per OTS molecule have been calculated through contact area analysis. Furthermore, the contrast of friction force images measured in contact mode was reversed to the 1LG/2LG adhesion contrast, and its origin was discussed in terms of the likely water depletion over hydrophobic domains as well as deformation in the contact area between the atomic force microscope tip and 1LG.
Figure S1 | Surface potential line profiles across gate and channel. (a) V CPD and (b) ΔV CPD = V... more Figure S1 | Surface potential line profiles across gate and channel. (a) V CPD and (b) ΔV CPD = V CPD (V g ) -V CPD (0) line profiles across gate-SiC-channel-SiC-gate along the dotted line indicated in the inset (a) for V g = 0, ±1 and ±2 V obtained using FM-KPFM for device #1. The inset in (a) shows a schematic of the device. The vertical dashed lines mark out the graphene-SiC edges. The black boxes mark out the area of zoom for Figures 2a and 2b (main text).
We investigate the local surface potential and Raman characteristics of as-grown and ex-situ hydr... more We investigate the local surface potential and Raman characteristics of as-grown and ex-situ hydrogen intercalated quasi-free standing graphene on 4H-SiC(0001) grown by chemical vapor deposition. Upon intercalation, transport measurements reveal a change in the carrier type from n- to p-type, accompanied by a more than three-fold increase in carrier mobility, up to μh ≈ 4540 cm(2) V(-1) s(-1). On a local scale, Kelvin probe force microscopy provides a complete and detailed map of the surface potential distribution of graphene domains of different thicknesses. Rearrangement of graphene layers upon intercalation to (n + 1)LG, where n is the number of graphene layers (LG) before intercalation, is demonstrated. This is accompanied by a significant increase in the work function of the graphene after the H2-intercalation, which confirms the change of majority carriers from electrons to holes. Raman spectroscopy and mapping corroborate surface potential studies.
We compare the three most commonly used scanning probe techniques to obtain a reliable value of t... more We compare the three most commonly used scanning probe techniques to obtain a reliable value of the work function in graphene domains of different thickness. The surface potential (SP) of graphene is directly measured in Hall bar geometry via a combination of electrical functional microscopy and spectroscopy techniques, which enables calibrated work function measurements of graphene domains in ambient conditions with values W 1LG ,4.55 6 0.02 eV and W 2LG , 4.44 6 0.02 eV for single-and bi-layer, respectively. We demonstrate that frequency-modulated Kelvin probe force microscopy (FM-KPFM) provides more accurate measurement of the SP than amplitude-modulated (AM)-KPFM. The discrepancy between experimental results obtained by different techniques is discussed. In addition, we use FM-KPFM for contactless measurements of the specific components of the device resistance. We show a strong non-Ohmic behavior of the electrode-graphene contact resistance and extract the graphene channel resistivity.
We demonstrate the temperature-mediated and completely reversible process of desorption-readsorpt... more We demonstrate the temperature-mediated and completely reversible process of desorption-readsorption of water on a few layers of epitaxial graphene on a 4H-SiC(0001) substrate. We show that under ambient conditions water forms solid structures on top of the second and third layers of graphene. In the case of strained or highly defective graphene domains, these features produce strongly correlated and reproducible patterns, implying importance of the underlying defects for the initial stages of water adsorption. Hydrophobicity increases with number of graphene layers.
2012 12th IEEE International Conference on Nanotechnology (IEEE-NANO), 2012
ABSTRACT Electrostatic Force Spectroscopy and Scanning Kelvin Probe Microscopy techniques are use... more ABSTRACT Electrostatic Force Spectroscopy and Scanning Kelvin Probe Microscopy techniques are used to study the performance of side-gated Hall devices made of epitaxial graphene on 4H-SiC(0001). Electrostatic Force Spectroscopy is a novel method which allows quantitative surface potential measurements with high spatial resolution. Using these techniques, we calibrate work function of the metal coated tip and define the work functions for single and double-layer graphene. We also show that the use of moderate strength electrical fields in the side-gate geometry does not notably change the performance of the device.
The temperature dependent resistance and the noise characteristics of an individual multiwall car... more The temperature dependent resistance and the noise characteristics of an individual multiwall carbon nanotube (CNT) decorated with a finite number of magnetic nanocubes are investigated. We show that CNT is a highly sensitive bolometer and can enable measurements of magnetic resonance in a single nanoparticle.
Using local scanning electrical techniques we study edge effects in side-gated Hall bar nanodevic... more Using local scanning electrical techniques we study edge effects in side-gated Hall bar nanodevices made of epitaxial graphene. We demonstrate that lithographically defined edges of the graphene channel exhibit hole conduction within the narrow band of ,60-125 nm width, whereas the bulk of the material is electron doped. The effect is the most pronounced when the influence of atmospheric contamination is minimal. We also show that the electronic properties at the edges can be precisely tuned from hole to electron conduction by using moderate strength electrical fields created by side-gates. However, the central part of the channel remains relatively unaffected by the side-gates and retains the bulk properties of graphene. G raphene is a two-dimensional material comprising carbon atoms closely packed in a honeycomb crystal structure 1 . Local layer thickness, substrate and environmental doping influence the material's nanoscale electronic properties. Due to the two-dimensional nature of graphene scanning probes can easily access its electronic states 2,3 ; on the other hand the properties of the material are easily affected by the surrounding environment.
We present experimental studies of both local and macroscopic electrical effects in uniform singl... more We present experimental studies of both local and macroscopic electrical effects in uniform single-(1LG) and bi-layer graphene (2LG) devices as well as in devices with non-uniform graphene coverage, under ambient conditions. DC transport measurements on sub-micron scale Hall bar devices were used to show a linear rise in carrier density with increasing 2LG coverage. Electrical scanning gate microscopy was used to locally top gate uniform and non-uniform devices in order to observe the effect of local electrical gating. We experimentally show a significant level of electric field screening by 2LG. We demonstrate that SGM technique is an extremely useful research tool for studies of local screening effects, which provides a complementary view on phenomena that are usually considered only within a macroscopic experimental scheme.
We report the use of an ultralow noise nano-superconducting quantum interference device (nanoSQUI... more We report the use of an ultralow noise nano-superconducting quantum interference device (nanoSQUID) to measure the hysteretic magnetization behavior of a single FePt nanobead at a temperature of around 7 K in a magnetic field of only ~10 mT. We also show that the nanobead can be accurately positioned with respect to the SQUID loop and then removed without affecting
Large area superconducting quantum interference devices (SQUIDs) are known to be ultimate sensors... more Large area superconducting quantum interference devices (SQUIDs) are known to be ultimate sensors of magnetic flux. By diminishing the size of a SQUID loop, the energy sensitivity of the device can be increased to near quantum-limited operation whilst also making it less sensitive to external magnetic field noise and improving its coupling to any nano-scale magnetic particle within the loop.
This paper deals with the modeling of sensitivity of epitaxial graphene Hall bars, from sub-micro... more This paper deals with the modeling of sensitivity of epitaxial graphene Hall bars, from sub-micrometer to micrometer size, to the stray field generated by a magnetic microbead. To demonstrate experiment feasibility, the model is first validated by comparison to measurement results, considering an ac-dc detection scheme. Then, a comprehensive numerical analysis is performed to investigate signal detriment caused by sensor material heterogeneities, saturation of bead magnetization at high fields, increment of bead distance from sensor surface, and device width increase.
This article addresses the much debated question whether the degree of hydrophobicity of single-l... more This article addresses the much debated question whether the degree of hydrophobicity of single-layer graphene (1LG) is different from that of double-layer graphene (2LG). Knowledge of the water affinity of graphene and its spatial variations is critically important as it can affect the graphene properties as well as the performance of graphene devices exposed to humidity. By employing chemical force microscopy with a probe rendered hydrophobic by functionalization with octadecyltrichlorosilane (OTS), the adhesion force between the probe and epitaxial graphene on SiC has been measured in deionized water. Owing to the hydrophobic attraction, a larger adhesion force was measured on 2LG Bernal-stacked domains of graphene surfaces, thus showing that 2LG is more hydrophobic than 1LG. Identification of 1LG and 2LG domains was achieved through Kelvin probe force microscopy and Raman spectral mapping. Approximate values of the adhesion force per OTS molecule have been calculated through contact area analysis. Furthermore, the contrast of friction force images measured in contact mode was reversed to the 1LG/2LG adhesion contrast, and its origin was discussed in terms of the likely water depletion over hydrophobic domains as well as deformation in the contact area between the atomic force microscope tip and 1LG.
Figure S1 | Surface potential line profiles across gate and channel. (a) V CPD and (b) ΔV CPD = V... more Figure S1 | Surface potential line profiles across gate and channel. (a) V CPD and (b) ΔV CPD = V CPD (V g ) -V CPD (0) line profiles across gate-SiC-channel-SiC-gate along the dotted line indicated in the inset (a) for V g = 0, ±1 and ±2 V obtained using FM-KPFM for device #1. The inset in (a) shows a schematic of the device. The vertical dashed lines mark out the graphene-SiC edges. The black boxes mark out the area of zoom for Figures 2a and 2b (main text).
We investigate the local surface potential and Raman characteristics of as-grown and ex-situ hydr... more We investigate the local surface potential and Raman characteristics of as-grown and ex-situ hydrogen intercalated quasi-free standing graphene on 4H-SiC(0001) grown by chemical vapor deposition. Upon intercalation, transport measurements reveal a change in the carrier type from n- to p-type, accompanied by a more than three-fold increase in carrier mobility, up to μh ≈ 4540 cm(2) V(-1) s(-1). On a local scale, Kelvin probe force microscopy provides a complete and detailed map of the surface potential distribution of graphene domains of different thicknesses. Rearrangement of graphene layers upon intercalation to (n + 1)LG, where n is the number of graphene layers (LG) before intercalation, is demonstrated. This is accompanied by a significant increase in the work function of the graphene after the H2-intercalation, which confirms the change of majority carriers from electrons to holes. Raman spectroscopy and mapping corroborate surface potential studies.
We compare the three most commonly used scanning probe techniques to obtain a reliable value of t... more We compare the three most commonly used scanning probe techniques to obtain a reliable value of the work function in graphene domains of different thickness. The surface potential (SP) of graphene is directly measured in Hall bar geometry via a combination of electrical functional microscopy and spectroscopy techniques, which enables calibrated work function measurements of graphene domains in ambient conditions with values W 1LG ,4.55 6 0.02 eV and W 2LG , 4.44 6 0.02 eV for single-and bi-layer, respectively. We demonstrate that frequency-modulated Kelvin probe force microscopy (FM-KPFM) provides more accurate measurement of the SP than amplitude-modulated (AM)-KPFM. The discrepancy between experimental results obtained by different techniques is discussed. In addition, we use FM-KPFM for contactless measurements of the specific components of the device resistance. We show a strong non-Ohmic behavior of the electrode-graphene contact resistance and extract the graphene channel resistivity.
We demonstrate the temperature-mediated and completely reversible process of desorption-readsorpt... more We demonstrate the temperature-mediated and completely reversible process of desorption-readsorption of water on a few layers of epitaxial graphene on a 4H-SiC(0001) substrate. We show that under ambient conditions water forms solid structures on top of the second and third layers of graphene. In the case of strained or highly defective graphene domains, these features produce strongly correlated and reproducible patterns, implying importance of the underlying defects for the initial stages of water adsorption. Hydrophobicity increases with number of graphene layers.
2012 12th IEEE International Conference on Nanotechnology (IEEE-NANO), 2012
ABSTRACT Electrostatic Force Spectroscopy and Scanning Kelvin Probe Microscopy techniques are use... more ABSTRACT Electrostatic Force Spectroscopy and Scanning Kelvin Probe Microscopy techniques are used to study the performance of side-gated Hall devices made of epitaxial graphene on 4H-SiC(0001). Electrostatic Force Spectroscopy is a novel method which allows quantitative surface potential measurements with high spatial resolution. Using these techniques, we calibrate work function of the metal coated tip and define the work functions for single and double-layer graphene. We also show that the use of moderate strength electrical fields in the side-gate geometry does not notably change the performance of the device.
The temperature dependent resistance and the noise characteristics of an individual multiwall car... more The temperature dependent resistance and the noise characteristics of an individual multiwall carbon nanotube (CNT) decorated with a finite number of magnetic nanocubes are investigated. We show that CNT is a highly sensitive bolometer and can enable measurements of magnetic resonance in a single nanoparticle.
Using local scanning electrical techniques we study edge effects in side-gated Hall bar nanodevic... more Using local scanning electrical techniques we study edge effects in side-gated Hall bar nanodevices made of epitaxial graphene. We demonstrate that lithographically defined edges of the graphene channel exhibit hole conduction within the narrow band of ,60-125 nm width, whereas the bulk of the material is electron doped. The effect is the most pronounced when the influence of atmospheric contamination is minimal. We also show that the electronic properties at the edges can be precisely tuned from hole to electron conduction by using moderate strength electrical fields created by side-gates. However, the central part of the channel remains relatively unaffected by the side-gates and retains the bulk properties of graphene. G raphene is a two-dimensional material comprising carbon atoms closely packed in a honeycomb crystal structure 1 . Local layer thickness, substrate and environmental doping influence the material's nanoscale electronic properties. Due to the two-dimensional nature of graphene scanning probes can easily access its electronic states 2,3 ; on the other hand the properties of the material are easily affected by the surrounding environment.
We present experimental studies of both local and macroscopic electrical effects in uniform singl... more We present experimental studies of both local and macroscopic electrical effects in uniform single-(1LG) and bi-layer graphene (2LG) devices as well as in devices with non-uniform graphene coverage, under ambient conditions. DC transport measurements on sub-micron scale Hall bar devices were used to show a linear rise in carrier density with increasing 2LG coverage. Electrical scanning gate microscopy was used to locally top gate uniform and non-uniform devices in order to observe the effect of local electrical gating. We experimentally show a significant level of electric field screening by 2LG. We demonstrate that SGM technique is an extremely useful research tool for studies of local screening effects, which provides a complementary view on phenomena that are usually considered only within a macroscopic experimental scheme.
Uploads
Papers by Olga Kazakova