Abstract
As conductors in electronic applications shrink, microscopic conduction processes lead to strong deviations from Ohm’s law. Depending on the length scales of momentum conserving (
l
MC
) and ...relaxing (
l
MR
) electron scattering, and the device size (
d
), current flows may shift from ohmic to ballistic to hydrodynamic regimes. So far, an in situ methodology to obtain these parameters within a micro/nanodevice is critically lacking. In this context, we exploit Sondheimer oscillations, semi-classical magnetoresistance oscillations due to helical electronic motion, as a method to obtain
l
MR
even when
l
MR
≫
d
. We extract
l
MR
from the Sondheimer amplitude in WP
2
, at temperatures up to
T
~ 40 K, a range most relevant for hydrodynamic transport phenomena. Our data on
μ
m-sized devices are in excellent agreement with experimental reports of the bulk
l
MR
and confirm that WP
2
can be microfabricated without degradation. These results conclusively establish Sondheimer oscillations as a quantitative probe of
l
MR
in micro-devices.
This work reports on a method to open nanoscale gaps in h-shaped graphene nano-constrictions by electrical breakdown at room temperature and pressure below 10−5 mbar. The method was validated on 275 ...devices, fabricated on eight different chips, using Chemical Vapor Deposition (CVD)-grown graphene from in-house production and from two commercial sources. The gap width was estimated by fitting the I–V traces after electrical breakdown with the Simmons model for the intermediate-voltage range. The statistics on the collected data demonstrates that the method results in normally distributed nanoscale gaps in h-shaped graphene nano-constrictions, with an estimated average width centered around 1 nm and a gap fabrication yield of 95%.
Graphene is an excellent 2D material for vertical organic transistors electrodes due to its weak electrostatic screening and field-tunable work function, in addition to its high conductivity, ...flexibility and optical transparency. Nevertheless, the interaction between graphene and other carbon-based materials, including small organic molecules, can affect the graphene electrical properties and therefore, the device performances. This work investigates the effects of thermally evaporated C60 (n-type) and Pentacene (p-type) thin films on the in-plane charge transport properties of large area CVD graphene under vacuum. This study was performed on a population of 300 graphene field effect transistors. The output characteristic of the transistors revealed that a C60 thin film adsorbate increased the graphene hole density by (1.65 ± 0.36) × 1012 cm−2, whereas a Pentacene thin film increased the graphene electron density by (0.55 ± 0.54) × 1012 cm−2. Hence, C60 induced a graphene Fermi energy downshift of about 100 meV, while Pentacene induced a Fermi energy upshift of about 120 meV. In both cases, the increase in charge carriers was accompanied by a reduced charge mobility, which resulted in a larger graphene sheet resistance of about 3 kΩ at the Dirac point. Interestingly, the contact resistance, which varied in the range 200 Ω–1 kΩ, was not significantly affected by the deposition of the organic molecules.
We report a study on the relationship between structure and electron transport properties of nanoscale graphene/pentacene interfaces. We fabricated graphene/pentacene interfaces from 10-30 nm thick ...needle-like pentacene nanostructures down to two-three layers (2L-3L) dendritic pentacene islands, and we measured their electron transport properties by conductive atomic force microscopy (C-AFM). The energy barrier at the interfaces, i.e. the energy position of the pentacene highest occupied molecular orbital (HOMO) with respect to the Fermi energy of the graphene and the C-AFM metal tip, are determined and discussed with the appropriate electron transport model (double Schottky diode model and Landauer-Buttiker model, respectively) taking into account the voltage-dependent charge doping of graphene. In both types of samples, the energy barrier at the graphene/pentacene interface is slightly larger than that at the pentacene/metal tip interface, resulting in 0.47-0.55 eV and 0.21-0.34 eV, respectively, for the 10-30 nm thick needle-like pentacene islands, and in 0.92-1.44 eV and 0.67-1.05 eV, respectively, for the 2L-3L thick dendritic pentacene nanostructures. We attribute this difference to the molecular organization details of the pentacene/graphene heterostructures, with pentacene molecules lying flat on the graphene in the needle-like pentacene nansotructures, while standing upright in 2L-3L dendritic islands, as observed from Raman spectroscopy.
Hybrid van der Waals heterostructures based on 2D materials and/or organic thin films are being evaluated as potential functional devices for a variety of applications. In this context, the ...graphene/organic semiconductor (Gr/OSC) heterostructure could represent the core element to build future vertical organic transistors based on two back-to-back Gr/OSC diodes sharing a common graphene sheet, which functions as the base electrode. However, the assessment of the Gr/OSC potential still requires a deeper understanding of the charge carrier transport across the interface as well as the development of wafer-scale fabrication methods. This work investigates the charge injection and transport across Au/OSC/Gr vertical heterostructures, focusing on poly(3-hexylthiophen-2,5-diyl) as the OSC, where the PMMA-free graphene layer functions as the top electrode. The structures are fabricated using a combination of processes widely exploited in semiconductor manufacturing and therefore are suited for industrial upscaling. Temperature-dependent current–voltage measurements and impedance spectroscopy show that the charge transport across both device interfaces is injection-limited by thermionic emission at high bias, while it is space charge limited at low bias, and that the P3HT can be assumed fully depleted in the high bias regime. From the space charge limited model, the out-of-plane charge carrier mobility in P3HT is found to be equal to μ ≈ 2.8 × 10–4 cm2 V–1 s–1, similar to the in-plane mobility reported in previous works, while the charge carrier density is N 0 ≈ 1.16 × 1015 cm–3, also in agreement with previously reported values. From the thermionic emission model, the energy barriers at the Gr/P3HT and Au/P3HT interfaces result in 0.30 eV and 0.25 eV, respectively. Based on the measured barriers heights, the energy band diagram of the vertical heterostructure is proposed under the hypothesis that P3HT is fully depleted.
We report a study on the relationship between the structure and electron transport properties of nanoscale graphene/pentacene interfaces. We fabricated graphene/pentacene interfaces from 10 to 30 nm ...thick needle-like pentacene nanostructures down to two-three layer (2L-3L) dendritic pentacene islands, and we measured their electron transport properties by conductive atomic force microscopy (C-AFM). The energy barrier at the interfaces,
i.e.
, the energy position of the pentacene highest occupied molecular orbital (HOMO) with respect to the Fermi energy of graphene and the C-AFM metal tip was determined and discussed with an appropriate electron transport model (a double Schottky diode model and a Landauer-Buttiker model, respectively) taking into account the voltage-dependent charge doping of graphene. In both types of samples, the energy barrier at the graphene/pentacene interface is slightly larger than that at the pentacene/metal tip interface, resulting in 0.47-0.55 eV and 0.21-0.34 eV, respectively, for the 10-30 nm thick needle-like pentacene islands, and 0.92-1.44 eV and 0.67-1.05 eV, respectively, for the 2L-3L thick dendritic pentacene nanostructures. We attribute this difference to the molecular organization details of the pentacene/graphene heterostructures, with pentacene molecules lying flat on graphene in the needle-like pentacene nanostructures, while standing upright in the 2L-3L dendritic islands, as observed from Raman spectroscopy.
The energetics of graphene/pentacene nanostructures is determined by conductive-AFM measurements combined with analytical models and related to the organization of the pentacene molecules at the interface as deduced from micro-Raman spectroscopy.
Among the family of 2D materials, graphene is the ideal candidate as top or interlayer electrode for hybrid van der Waals heterostructures made of organic thin films and 2D materials due to its high ...conductivity and mobility and its inherent ability of forming neat interfaces without diffusing in the adjacent organic layer. Understanding the charge injection mechanism at graphene/organic semiconductor interfaces is therefore crucial to develop organic electronic devices. In particular, Gr/C60 interfaces are promising building blocks for future n-type vertical organic transistors exploiting graphene as tunneling base electrode in a two back-to-back Gr/C60 Schottky diode configuration. This work delves into the charge transport mechanism across Au/C60/Gr vertical heterostructures fabricated on Si/SiO2 using a combination of techniques commonly used in the semiconductor industry, where a resist-free CVD graphene layer functions as a top electrode. Temperature-dependent electrical measurements show that the transport mechanism is injection limited and occurs via Fowler–Nordheim tunneling at low temperature, while it is dominated by a nonideal thermionic emission at room and high temperatures, with energy barriers at room temperature of ca. 0.58 and 0.65 eV at the Gr/C60 and Au/C60 interfaces, respectively. Impedance spectroscopy confirms that the organic semiconductor is depleted, and the energy band diagram results in two electron blocking interfaces. The resulting rectifying nature of the Gr/C60 interface could be exploited in organic hot electron transistors and vertical organic permeable-base transistors.
Graphene is an excellent 2D material for vertical organic transistors electrodes due to its weak electrostatic screening and field-tunable work function, in addition to its high conductivity, ...flexibility and optical transparency. Nevertheless, the interaction between graphene and other carbon-based materials, including small organic molecules, can affect the graphene electrical properties and therefore, the device performances. This work investigates the effects of thermally evaporated C60 (
-type) and Pentacene (
-type) thin films on the in-plane charge transport properties of large area CVD graphene under vacuum. This study was performed on a population of 300 graphene field effect transistors. The output characteristic of the transistors revealed that a C60 thin film adsorbate increased the graphene hole density by (1.65 ± 0.36) × 10
cm
, whereas a Pentacene thin film increased the graphene electron density by (0.55 ± 0.54) × 10
cm
. Hence, C60 induced a graphene Fermi energy downshift of about 100 meV, while Pentacene induced a Fermi energy upshift of about 120 meV. In both cases, the increase in charge carriers was accompanied by a reduced charge mobility, which resulted in a larger graphene sheet resistance of about 3 kΩ at the Dirac point. Interestingly, the contact resistance, which varied in the range 200 Ω-1 kΩ, was not significantly affected by the deposition of the organic molecules.
Graphene is an excellent 2D material for vertical organic transistors electrodes due to its weak electrostatic screening and field-tunable work function, in addition to its high conductivity, ...flexibility and optical transparency. Nevertheless, the interaction between graphene and other carbon-based materials, including small organic molecules, can affect the graphene electrical properties and therefore, the device performances. This work investigates the effects of thermally evaporated C60 (n-type) and Pentacene (p-type) thin films on the in-plane charge transport properties of large area CVD graphene under vacuum. This study was performed on a population of 300 graphene field effect transistors. The output characteristic of the transistors revealed that a C60 thin film adsorbate increased the graphene hole density by (1.65 ± 0.36) × 10sup.12 cmsup.−2, whereas a Pentacene thin film increased the graphene electron density by (0.55 ± 0.54) × 10sup.12 cmsup.−2. Hence, C60 induced a graphene Fermi energy downshift of about 100 meV, while Pentacene induced a Fermi energy upshift of about 120 meV. In both cases, the increase in charge carriers was accompanied by a reduced charge mobility, which resulted in a larger graphene sheet resistance of about 3 kΩ at the Dirac point. Interestingly, the contact resistance, which varied in the range 200 Ω-1 kΩ, was not significantly affected by the deposition of the organic molecules.
As conductors in electronic applications shrink, microscopic conduction processes lead to strong deviations from Ohm's law. Depending on the length scales of momentum conserving (l
) and relaxing (l
...) electron scattering, and the device size (d), current flows may shift from ohmic to ballistic to hydrodynamic regimes. So far, an in situ methodology to obtain these parameters within a micro/nanodevice is critically lacking. In this context, we exploit Sondheimer oscillations, semi-classical magnetoresistance oscillations due to helical electronic motion, as a method to obtain l
even when l
≫ d. We extract l
from the Sondheimer amplitude in WP
, at temperatures up to T ~ 40 K, a range most relevant for hydrodynamic transport phenomena. Our data on μm-sized devices are in excellent agreement with experimental reports of the bulk l
and confirm that WP
can be microfabricated without degradation. These results conclusively establish Sondheimer oscillations as a quantitative probe of l
in micro-devices.
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