Suresh V.

Due to their extraordinary optical and electronic character at low-dimensionality, two-dimensional (2D) layered selenides and tellurides are an actively-pursued area both in fundamental research and in applications. Recently non-equilibrium growth of 2D layered materials using molecular beam epitaxy (MBE) has attracted widespread attention in the scientific community for realizing highly scaled technologies involving both novel device structures and new physics. Here we summarized our recent… Show more

Due to their extraordinary optical and electronic character at low-dimensionality, two-dimensional (2D) layered selenides and tellurides are an actively-pursued area both in fundamental research and in applications. Recently non-equilibrium growth of 2D layered materials using molecular beam epitaxy (MBE) has attracted widespread attention in the scientific community for realizing highly scaled technologies involving both novel device structures and new physics. Here we summarized our recent work on MBE growth of 2D layered selenides and tellurides, as well as their heterostructures, along with their characterization. This chapter also points out a number of challenges lying ahead in this field, and provides our viewpoint on what the future holds for MBE-grown 2D layered selenides and tellurides. Show less

Chemical functionalization is demonstrated to enhance the p-type electrical performance of two-dimensional (2D) layered tungsten diselenide (WSe2) field-effect transistors (FETs) using a one-step dipping process in an aqueous solution of ammonium sulfide [(NH4)2S(aq)]. Molecularly resolved scanning tunneling microscopy and spectroscopy reveal that molecular adsorption on a monolayer WSe2 surface induces a reduction of the electronic band gap from 2.1 to 1.1 eV and a Fermi level shift toward the… Show more

Chemical functionalization is demonstrated to enhance the p-type electrical performance of two-dimensional (2D) layered tungsten diselenide (WSe2) field-effect transistors (FETs) using a one-step dipping process in an aqueous solution of ammonium sulfide [(NH4)2S(aq)]. Molecularly resolved scanning tunneling microscopy and spectroscopy reveal that molecular adsorption on a monolayer WSe2 surface induces a reduction of the electronic band gap from 2.1 to 1.1 eV and a Fermi level shift toward the WSe2 valence band edge (VBE), consistent with an increase in the density of positive charge carriers. The mechanism of electronic transformation of WSe2 by (NH4)2S(aq) chemical treatment is elucidated using density functional theory calculations which reveal that molecular “SH” adsorption on the WSe2 surface introduces additional in-gap states near the VBE, thereby, inducing a Fermi level shift toward the VBE along with a reduction in the electronic band gap. As a result of the (NH4)2S(aq) chemical treatment, the p-branch ON-currents (ION) of back-gated few-layer ambipolar WSe2 FETs are enhanced by about 2 orders of magnitude, and a ∼6× increase in the hole field-effect mobility is observed, the latter primarily resulting from the p-doping-induced narrowing of the Schottky barrier width leading to an enhanced hole injection at the WSe2/contact metal interface. This (NH4)2S(aq) chemical functionalization technique can serve as a model method to control the electronic band structure and enhance the performance of devices based on 2D layered transition-metal dichalcogenides. Show less

Tin Diselenide (SnSe 2 ) is a two-dimensional layered crystal commonly found in octahedral coordination (1T phase). It has been reported to have a high electron affinity of around 5.1 eV and a bandgap of 1 eV [1-2], which can form staggered band alignment with tungsten diselenide (WSe 2 ) in Thin-TFETs [3]. However, its lack of gate modulation remains a mystery [4]. In this work, we investigate the Fermi level tunability of SnSe 2 by counter doping using a polymer electrolyte, PEO:CsClO 4… Show more

Tin Diselenide (SnSe 2 ) is a two-dimensional layered crystal commonly found in octahedral coordination (1T phase). It has been reported to have a high electron affinity of around 5.1 eV and a bandgap of 1 eV [1-2], which can form staggered band alignment with tungsten diselenide (WSe 2 ) in Thin-TFETs [3]. However, its lack of gate modulation remains a mystery [4]. In this work, we investigate the Fermi level tunability of SnSe 2 by counter doping using a polymer electrolyte, PEO:CsClO 4 . This counter doping technique increases the on/off ratio of SnSe 2 field effect transistor (FET) from 2 times to 50 times, a record high value. Meanwhile, a device model of SnSe 2 FET with ion doping and subgap density of states (DOS) has been proposed to fit the experimental data. The extracted effective number of acceptor-like subgap states is as high as 4.16 × 10 19 cm -3 (in comparison with near 5 × 10 17 cm -3 extracted for amorphous thin-film transistors [5]). This can explain the weak Fermi level tunability of SnSe 2 and direct future material development towards TFETs. Show less

SnSe2 is a layered main-group metal dichalcogenide that has exhibited gate-tunable interfacial superconductivity as well as promising optoelectronic applications. Here, we synthesize SnSe2 films by molecular beam epitaxy and investigate their electronic structure with angle-resolved photoemission spectroscopy (ARPES). A comparison between density functional theory calculations and ARPES data from a thick film reveals the importance of spin-orbit coupling and out-of-plane dispersion in the SnSe2… Show more

SnSe2 is a layered main-group metal dichalcogenide that has exhibited gate-tunable interfacial superconductivity as well as promising optoelectronic applications. Here, we synthesize SnSe2 films by molecular beam epitaxy and investigate their electronic structure with angle-resolved photoemission spectroscopy (ARPES). A comparison between density functional theory calculations and ARPES data from a thick film reveals the importance of spin-orbit coupling and out-of-plane dispersion in the SnSe2 valence bands, which were neglected in previous studies of its electronic structure. We conclude that the conduction band minimum lies along the M-L direction in momentum space, while the valence band maximum lies along Γ−K. Show less

Defect engineering of 2D semiconducting transition metal dichalcogenides (TMDCs) has been demonstrated to be a promising way to tune both their bandgaps and carrier concentrations. Moreover, controlled introduction of defects in the source/drain access regions of a TMDC FET can boost its performance by decreasing the contact resistance at the metal/TMDC interface [1]. While chemical functionalization offers a facile route towards defect engineering in 2D TMDCs, several chemically-treated TMDCs… Show more

Defect engineering of 2D semiconducting transition metal dichalcogenides (TMDCs) has been demonstrated to be a promising way to tune both their bandgaps and carrier concentrations. Moreover, controlled introduction of defects in the source/drain access regions of a TMDC FET can boost its performance by decreasing the contact resistance at the metal/TMDC interface [1]. While chemical functionalization offers a facile route towards defect engineering in 2D TMDCs, several chemically-treated TMDCs have not been fully understood at the molecular level. In this study, chemical sulfur treatment (ST) utilizing ammonium sulfide [(NH 4 ) 2 S] solution is shown to enhance the p-type behavior in 2D WSe 2 via introduction of acceptor defect states near its valence band edge (VBE), with the results verified using detailed scanning tunneling microscopy (STM)/spectroscopy (STS) studies, field-effect transistor (FET) measurements and theoretical density-of-states (DOS) calculations. Show less

Epitaxy is a process by which a thin layer of one crystal is deposited in an ordered fashion onto a substrate crystal. The direct epitaxial growth of semiconductor heterostructures on top of crystalline superconductors has proved challenging. Here, however, we report the successful use of molecular beam epitaxy to grow and integrate niobium nitride (NbN)-based superconductors with the wide-bandgap family of semiconductors—silicon carbide, gallium nitride (GaN) and aluminium gallium nitride… Show more

Epitaxy is a process by which a thin layer of one crystal is deposited in an ordered fashion onto a substrate crystal. The direct epitaxial growth of semiconductor heterostructures on top of crystalline superconductors has proved challenging. Here, however, we report the successful use of molecular beam epitaxy to grow and integrate niobium nitride (NbN)-based superconductors with the wide-bandgap family of semiconductors—silicon carbide, gallium nitride (GaN) and aluminium gallium nitride (AlGaN). We apply molecular beam epitaxy to grow an AlGaN/GaN quantum-well heterostructure directly on top of an ultrathin crystalline NbN superconductor. The resulting high-mobility, two-dimensional electron gas in the semiconductor exhibits quantum oscillations, and thus enables a semiconductor transistor—an electronic gain element—to be grown and fabricated directly on a crystalline superconductor. Using the epitaxial superconductor as the source load of the transistor, we observe in the transistor output characteristics a negative differential resistance—a feature often used in amplifiers and oscillators. Our demonstration of the direct epitaxial growth of high-quality semiconductor heterostructures and devices on crystalline nitride superconductors opens up the possibility of combining the macroscopic quantum effects of superconductors with the electronic, photonic and piezoelectric properties of the group III/nitride semiconductor family. Show less

SnSe2 is currently considered a potential two-dimensional material that can form a near-broken gap heterojunction in a tunnel field-effect transistor due to its large electron affinity which is experimentally confirmed in this letter. With the results from internal photoemission and angle-resolved photoemission spectroscopy performed on Al/Al2O3/SnSe2/GaAs and SnSe2/GaAs test structures where SnSe2 is grown on GaAs by molecular beam epitaxy, we ascertain a (5.2 ± 0.1) eV electron affinity of… Show more

SnSe2 is currently considered a potential two-dimensional material that can form a near-broken gap heterojunction in a tunnel field-effect transistor due to its large electron affinity which is experimentally confirmed in this letter. With the results from internal photoemission and angle-resolved photoemission spectroscopy performed on Al/Al2O3/SnSe2/GaAs and SnSe2/GaAs test structures where SnSe2 is grown on GaAs by molecular beam epitaxy, we ascertain a (5.2 ± 0.1) eV electron affinity of SnSe2. The band offset from the SnSe2 Fermi level to the Al2O3 conduction band minimum is found to be (3.3 ± 0.05) eV and SnSe2 is seen to have a high level of intrinsic electron (n-type) doping with the Fermi level positioned at about 0.2 eV above its conduction band minimum. It is concluded that the electron affinity of SnSe2 is larger than that of most semiconductors and can be combined with other appropriate semiconductors to form near broken-gap heterojunctions for the tunnel field-effect transistor that can potentially achieve high on-currents. Show less

MoTe2 is the least explored material in the Molybdenum-chalcogen family. Molecular beam epitaxy (MBE) provides a unique opportunity to tackle the small electronegativity difference between Mo and Te while growing layer by layer away from thermodynamic equilibrium. We find that for a few-layer MoTe2 grown at a moderate rate of ∼6 min per monolayer, a narrow window in temperature (above Te cell temperature) and Te:Mo ratio exists, where we can obtain pure phase 2H-MoTe2. This is confirmed using… Show more

MoTe2 is the least explored material in the Molybdenum-chalcogen family. Molecular beam epitaxy (MBE) provides a unique opportunity to tackle the small electronegativity difference between Mo and Te while growing layer by layer away from thermodynamic equilibrium. We find that for a few-layer MoTe2 grown at a moderate rate of ∼6 min per monolayer, a narrow window in temperature (above Te cell temperature) and Te:Mo ratio exists, where we can obtain pure phase 2H-MoTe2. This is confirmed using reflection high-energy electron diffraction (RHEED), Raman spectroscopy and X-ray photoemission spectroscopy (XPS). For growth on CaF2, Grazing incidence X-ray diffraction (GI-XRD) reveals a grain size of ∼90 Å and presence of twinned grains. In this work, we hypothesis the presence of excess Te incorporation in MBE grown few layer 2H-MoTe2. For film on CaF2, it is based on >2 Te:Mo stoichiometry using XPS as well as ‘a’ and ‘c’ lattice spacing greater than bulk 2H-MoTe2. On GaAs, its based on observations of Te crystallite formation on film surface, 2 × 2 superstructure observed in RHEED and low energy electron diffraction, larger than bulk c-lattice spacing as well as the lack of electrical conductivity modulation by field effect. Finally, thermal stability and air sensitivity of MBE 2H-MoTe2 is investigated by temperature dependent XRD and XPS, respectively. Show less

Layered materials have drawn intense attention lately due to the possibility of realizing highly scalable low power electronic and other novel devices. Although several proof-of-concept devices have been demonstrated recently with exfoliated micron-size flakes, controllable growth of large-area electronic grade material, essential for mass production, is in its infancy. Molecular beam epitaxy (MBE) holds the promise to allow large area, layer-controlled uniform growth and in-situ probing. To… Show more

Layered materials have drawn intense attention lately due to the possibility of realizing highly scalable low power electronic and other novel devices. Although several proof-of-concept devices have been demonstrated recently with exfoliated micron-size flakes, controllable growth of large-area electronic grade material, essential for mass production, is in its infancy. Molecular beam epitaxy (MBE) holds the promise to allow large area, layer-controlled uniform growth and in-situ probing. To this end, in this work we have summarized the literature in the field and fortified it with observations made by us on MBE grown materials like MoSe2, MoTe2, WSe2, SnSe2, MoSe2-xTex, their heterostructures and superlattices. We have summarized investigations on several substrates under different growth regimes that are being pursued in the community. This analysis sheds light on the strengths and limitations of various characterization techniques and the challenges to overcome defects in MBE 2D materials. Show less

Topological crystalline insulators have been recently predicted and observed in rock-salt structure SnSe {111} thin films. Previous studies have suggested that the Se-terminated surface of this thin film with hydrogen passivation has a reduced surface energy and is thus a preferred configuration. In this paper, synchrotron-based angle-resolved photoemission spectroscopy, along with density functional theory calculations, is used to demonstrate that a rock-salt SnSe {111} thin film epitaxially… Show more

Topological crystalline insulators have been recently predicted and observed in rock-salt structure SnSe {111} thin films. Previous studies have suggested that the Se-terminated surface of this thin film with hydrogen passivation has a reduced surface energy and is thus a preferred configuration. In this paper, synchrotron-based angle-resolved photoemission spectroscopy, along with density functional theory calculations, is used to demonstrate that a rock-salt SnSe {111} thin film epitaxially grown on Bi2Se3 has a stable Sn-terminated surface. These observations are supported by low-energy electron diffraction (LEED) intensity-voltage measurements and dynamical LEED calculations, which further show that the Sn-terminated SnSe {111} thin film has undergone a surface structural relaxation of the interlayer spacing between the Sn and Se atomic planes. In sharp contrast to the Se-terminated counterpart, the observed Dirac surface state in the Sn-terminated SnSe {111} thin film is shown to yield a high Fermi velocity, 0.50×10^6m/s, which suggests a potential mechanism of engineering the Dirac surface state of topological materials by tuning the surface configuration. Show less

To fabricate practical devices based on semiconducting two-dimensional (2D) materials, the source, channel, and drain materials are exposed to ambient air. However, the response of layered 2D materials to air has not been fully elucidated at the molecular level. In the present report, the effects of air exposure on transition metal dichalcogenides (TMD) and metal dichalcogenides (MD) are studied using ultrahigh-vacuum scanning tunneling microscopy (STM). The effects of a 1-day ambient air… Show more

To fabricate practical devices based on semiconducting two-dimensional (2D) materials, the source, channel, and drain materials are exposed to ambient air. However, the response of layered 2D materials to air has not been fully elucidated at the molecular level. In the present report, the effects of air exposure on transition metal dichalcogenides (TMD) and metal dichalcogenides (MD) are studied using ultrahigh-vacuum scanning tunneling microscopy (STM). The effects of a 1-day ambient air exposure on MBE-grown WSe2, chemical vapor deposition (CVD)-grown MoS2, and MBE SnSe2 are compared. Both MBE-grown WSe2 and CVD-grown MoS2 display a selective air exposure response at the step edges, consistent with oxidation on WSe2 and adsorption of hydrocarbon on MoS2, while the terraces and domain/grain boundaries of both TMDs are nearly inert to ambient air. Conversely, MBE-grown SnSe2, an MD, is not stable in ambient air. After exposure in ambient air for 1 day, the entire surface of SnSe2 is decomposed to SnOx and SeOx, as seen with X-ray photoelectron spectroscopy. Since the oxidation enthalpy of all three materials is similar, the data is consistent with greater oxidation of SnSe2 being driven by the weak bonding of SnSe2. Show less

To deposit an ultrathin dielectric onto WSe2, monolayer titanyl phthalocyanine (TiOPc) is deposited by molecular beam epitaxy as a seed layer for atomic layer deposition (ALD) of Al2O3 on WSe2. TiOPc molecules are arranged in a flat monolayer with 4-fold symmetry as measured by scanning tunneling microscopy. ALD pulses of trimethyl aluminum and H2O nucleate on the TiOPc, resulting in a uniform deposition of Al2O3, as confirmed by atomic force microscopy and cross-sectional transmission electron… Show more

To deposit an ultrathin dielectric onto WSe2, monolayer titanyl phthalocyanine (TiOPc) is deposited by molecular beam epitaxy as a seed layer for atomic layer deposition (ALD) of Al2O3 on WSe2. TiOPc molecules are arranged in a flat monolayer with 4-fold symmetry as measured by scanning tunneling microscopy. ALD pulses of trimethyl aluminum and H2O nucleate on the TiOPc, resulting in a uniform deposition of Al2O3, as confirmed by atomic force microscopy and cross-sectional transmission electron microscopy. The field-effect transistors (FETs) formed using this process have a leakage current of 0.046 pA/μm2 at 1 V gate bias with 3.0 nm equivalent oxide thickness, which is a lower leakage current than prior reports. The n-branch of the FET yielded a subthreshold swing of 80 mV/decade. Show less

We report the covalent bonding enabled modulation of the interfacial thermal conductance between graphene and metals Cu, Al, and Pt by controlling the oxidation of graphene. By combining comprehensive X-ray photoelectron spectroscopy (XPS) analysis and time-domain thermoreflectance measurements, we quantify the effect of graphene oxidation on interfacial thermal conductance. It was found that thermal conductance increases with the degree of graphene oxidation until a peak value is obtained at… Show more

We report the covalent bonding enabled modulation of the interfacial thermal conductance between graphene and metals Cu, Al, and Pt by controlling the oxidation of graphene. By combining comprehensive X-ray photoelectron spectroscopy (XPS) analysis and time-domain thermoreflectance measurements, we quantify the effect of graphene oxidation on interfacial thermal conductance. It was found that thermal conductance increases with the degree of graphene oxidation until a peak value is obtained at an oxygen/carbon atom percentage of ∼7.7%. The maximum enhancement in thermal conductance was measured to be 55%, 38%, and 49% for interfaces between oxidized graphene and Cu, Al, and Pt, respectively. In situ XPS measurements show that oxygen covalently binds to Cu and graphene simultaneously, forming a highly efficient bridge to enhance the thermal transport. Our molecular dynamics simulations verify that strong interfacial covalent bonds are the key to the thermal conductance enhancement. This work provides valuable insights into the mechanism of functionalization-induced thermal conductance enhancement and design guidelines for graphene-based devices. Show less

The effect of air exposure on 2H-WSe2/HOPG is determined via scanning tunneling microscopy (STM). WSe2 was grown by molecular beam epitaxy on highly oriented pyrolytic graphite (HOPG), and afterward, a Se adlayer was deposited in situ on WSe2/HOPG to prevent unintentional oxidation during transferring from the growth chamber to the STM chamber. After annealing at 773 K to remove the Se adlayer, STM images show that WSe2 layers nucleate at both step edges and terraces of the HOPG. Exposure to… Show more

The effect of air exposure on 2H-WSe2/HOPG is determined via scanning tunneling microscopy (STM). WSe2 was grown by molecular beam epitaxy on highly oriented pyrolytic graphite (HOPG), and afterward, a Se adlayer was deposited in situ on WSe2/HOPG to prevent unintentional oxidation during transferring from the growth chamber to the STM chamber. After annealing at 773 K to remove the Se adlayer, STM images show that WSe2 layers nucleate at both step edges and terraces of the HOPG. Exposure to air for 1 week and 9 weeks caused air-induced adsorbates to be deposited on the WSe2 surface; however, the band gap of the terraces remained unaffected and nearly identical to those on decapped WSe2. The air-induced adsorbates can be removed by annealing at 523 K. In contrast to WSe2 terraces, air exposure caused the edges of the WSe2 to oxidize and form protrusions, resulting in a larger band gap in the scanning tunneling spectra compared to the terraces of air-exposed WSe2 monolayers. The preferential oxidation at the WSe2 edges compared to the terraces is likely the result of dangling edge bonds. In the absence of air exposure, the dangling edge bonds had a smaller band gap compared to the terraces and a shift of about 0.73 eV in the Fermi level toward the valence band. However, after air exposure, the band gap of the oxidized WSe2 edges became about 1.08 eV larger than that of the WSe2 terraces, resulting in the electronic passivation of the WSe2. Show less

Layered materials are an actively pursued area of research for realizing highly scaled technologies involving both traditional device structures as well as new physics. Lately, non-equilibrium growth of 2D materials using molecular beam epitaxy (MBE) is gathering traction in the scientific community and here we aim to highlight one of its strengths, growth of abrupt heterostructures, and superlattices (SLs). In this work we present several of the firsts: first growth of MoTe2 by MBE, MoSe2 on… Show more

Layered materials are an actively pursued area of research for realizing highly scaled technologies involving both traditional device structures as well as new physics. Lately, non-equilibrium growth of 2D materials using molecular beam epitaxy (MBE) is gathering traction in the scientific community and here we aim to highlight one of its strengths, growth of abrupt heterostructures, and superlattices (SLs). In this work we present several of the firsts: first growth of MoTe2 by MBE, MoSe2 on Bi2Se3 SLs, transition metal dichalcogenide (TMD) SLs, and lateral junction between a quintuple atomic layer of Bi2Te3 and a triple atomic layer of MoTe2. Reflected high electron energy diffraction oscillations presented during the growth of TMD SLs strengthen our claim that ultrathin heterostructures with monolayer layer control is within reach. Show less

Most III-nitride semiconductors are grown on non-lattice-matched substrates like sapphire or silicon due to the extreme difficulty of obtaining a native GaN substrate. We show that several layered transition-metal dichalcogenides are closely lattice-matched to GaN and report the growth of GaN on a range of such layered materials. We report detailed studies of the growth of GaN on mechanically-exfoliated flakes WS2 and MoS2 by metalorganic vapour phase epitaxy. Structural and optical… Show more

Most III-nitride semiconductors are grown on non-lattice-matched substrates like sapphire or silicon due to the extreme difficulty of obtaining a native GaN substrate. We show that several layered transition-metal dichalcogenides are closely lattice-matched to GaN and report the growth of GaN on a range of such layered materials. We report detailed studies of the growth of GaN on mechanically-exfoliated flakes WS2 and MoS2 by metalorganic vapour phase epitaxy. Structural and optical characterization show that strain-free, single-crystal islands of GaN are obtained on the underlying chalcogenide flakes. We obtain strong near-band-edge emission from these layers and analyse their temperature-dependent photoluminescence properties. We also report a proof-of-concept demonstration of large-area growth of GaN on CVD MoS2. Our results show that the transition-metal dichalcogenides can serve as novel near-lattice-matched substrates for nitride growth. Show less

We discuss growth and magnetic properties of high-quality two dimensional (2D) Sn1−xMnxSe2 films. Thin films of this 2D ternary alloy with a wide range of Mn concentrations were successfully grown by molecular beam epitaxy. Mn concentrations up to x ≈ 0.60 were achieved without destroying the crystal structure of the parent SnSe2 2D system. Most important, the specimens show clear weak ferromagnetic behavior above room temperature, which should be of interest for 2D spintronic applications.

We report the structural and optical properties of a molecular beam epitaxy (MBE) grown 2-dimensional (2D) material molybdenum diselenide (MoSe 2) on graphite, CaF 2 and epitaxial graphene. Extensive characterizations reveal that 2H–MoSe 2 grows by van-der-Waals epitaxy on all three substrates with a preferred crystallographic orientation and a Mo: Se ratio of 1: 2. Photoluminescence at room temperature (~ 1.56 eV) is observed in monolayer MoSe 2 on both CaF 2 and epitaxial graphene. The band… Show more

We report the structural and optical properties of a molecular beam epitaxy (MBE) grown 2-dimensional (2D) material molybdenum diselenide (MoSe 2) on graphite, CaF 2 and epitaxial graphene. Extensive characterizations reveal that 2H–MoSe 2 grows by van-der-Waals epitaxy on all three substrates with a preferred crystallographic orientation and a Mo: Se ratio of 1: 2. Photoluminescence at room temperature (~ 1.56 eV) is observed in monolayer MoSe 2 on both CaF 2 and epitaxial graphene. The band edge absorption is very sharp,< 60 meV over three decades. Overcoming the observed small grains by promoting mobility of Mo adatoms would make MBE a powerful technique to achieve high quality 2D materials and heterostructures. Show less

We report on the fabrication and characterization of synthesized multiwall MoS2 nanotube (NT) and nanoribbon (NR) field-effect transistors (FETs). The MoS2 NTs and NRs were grown by chemical transport, using iodine as a transport agent. Raman spectroscopy confirms the material as unambiguously MoS2 in NT, NR, and flake forms. Transmission electron microscopy was used to observe cross sections of the devices after electrical measurements and these were used in the interpretation of the… Show more

We report on the fabrication and characterization of synthesized multiwall MoS2 nanotube (NT) and nanoribbon (NR) field-effect transistors (FETs). The MoS2 NTs and NRs were grown by chemical transport, using iodine as a transport agent. Raman spectroscopy confirms the material as unambiguously MoS2 in NT, NR, and flake forms. Transmission electron microscopy was used to observe cross sections of the devices after electrical measurements and these were used in the interpretation of the electrical measurements, allowing the estimation of the current density. The NT and NR FETs demonstrate n-type behavior, with ON/OFF current ratios exceeding 103, and with current densities of 1.02 μA/μm and 0.79 μA/μm at VDS = 0.3 V and VBG = 1 V, respectively. Photocurrent measurements conducted on a MoS2 NT FET revealed short-circuit photocurrent of tens of nanoamps under an excitation optical power of 78 μW and 488 nm wavelength, which corresponds to a responsivity of 460 μA/W. A long channel transistor model was used to model the common-source characteristics of MoS2 NT and NR FETs and was shown to be consistent with the measured data. Show less

The properties of multilayer exfoliated MoTe2 field-effect transistors (FETs) on SiO2 were investigated for channel thicknesses from 6 to 44 monolayers (MLs). All transistors showed p-type conductivity at zero back-gate bias. For channel thicknesses of 8 ML or less, the transistors exhibited ambipolar characteristics. ON/OFF current ratio was greatest, 1  × 105, for the transistor with the thinnest channel, 6 ML. Devices showed a clear photoresponse to wavelengths between 510 and 1080 nm at… Show more

The properties of multilayer exfoliated MoTe2 field-effect transistors (FETs) on SiO2 were investigated for channel thicknesses from 6 to 44 monolayers (MLs). All transistors showed p-type conductivity at zero back-gate bias. For channel thicknesses of 8 ML or less, the transistors exhibited ambipolar characteristics. ON/OFF current ratio was greatest, 1  × 105, for the transistor with the thinnest channel, 6 ML. Devices showed a clear photoresponse to wavelengths between 510 and 1080 nm at room temperature. Temperature-dependent current-voltage measurements were performed on a FET with 30 layers of MoTe2. When the channel is turned-on and p-type, the temperature dependence is barrier-limited by the Au/Ti/MoTe2 contact with a hole activation energy of 0.13 eV. A long channel transistor model with Schottky barrier contacts is shown to be consistent with the common-source characteristics. Show less