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1.
Artigo em Inglês | MEDLINE | ID: mdl-32192325

RESUMO

Doping is a key process by which the concentration and type of majority carriers can be tuned to achieve desired conduction properties. The common way of doping is via bulk impurities, as in the case of silicon. For van der Waals bonded semiconductors, control over bulk impurities is not as well developed, because they may either migrate between the layers or bond with the surfaces or interfaces becoming undesired scattering centers for carriers. Herein, we investigate by means of Kelvin probe force microscopy (KPFM) and density functional theory calculations (DFT) the doping of MoTe2 via surface charge transfer occurring in air. Using DFT, we show that oxygen molecules physisorb on the surface and increase its work function (compared to pristine surfaces) toward p-type behavior, which is consistent with our KPFM measurements. The surface charge transfer doping (SCTD) driven by adsorbed oxygen molecules can be easily controlled or reversed through thermal annealing of the entire sample. Furthermore, we also demonstrate local control of the doping by contact electrification. As a reversible and controllable nanoscale physisorption process, SCTD can thus open new avenues for the emerging field of 2D electronics.

2.
Artigo em Inglês | MEDLINE | ID: mdl-31555014
3.
Sci Adv ; 5(8): eaax1325, 2019 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-31453337

RESUMO

Heterogeneous integration of nanomaterials has enabled advanced electronics and photonics applications. However, similar progress has been challenging for thermal applications, in part due to shorter wavelengths of heat carriers (phonons) compared to electrons and photons. Here, we demonstrate unusually high thermal isolation across ultrathin heterostructures, achieved by layering atomically thin two-dimensional (2D) materials. We realize artificial stacks of monolayer graphene, MoS2, and WSe2 with thermal resistance greater than 100 times thicker SiO2 and effective thermal conductivity lower than air at room temperature. Using Raman thermometry, we simultaneously identify the thermal resistance between any 2D monolayers in the stack. Ultrahigh thermal isolation is achieved through the mismatch in mass density and phonon density of states between the 2D layers. These thermal metamaterials are an example in the emerging field of phononics and could find applications where ultrathin thermal insulation is desired, in thermal energy harvesting, or for routing heat in ultracompact geometries.

4.
ACS Appl Mater Interfaces ; 11(38): 35389-35393, 2019 Sep 25.
Artigo em Inglês | MEDLINE | ID: mdl-31468959

RESUMO

Thermal annealing of Ti contacts is commonly implemented in the fabrication of MoS2 devices; however, its effects on interface chemistry have not been previously reported in the literature. In this work, the thermal stability of titanium contacts deposited on geological bulk single crystals of MoS2 in ultrahigh vacuum (UHV) is investigated with X-ray photoelectron spectroscopy and scanning transmission electron microscopy (STEM). In the as-deposited condition, the reaction of Ti with MoS2 is observed resulting in a diffuse interface between the two materials that comprises metallic molybdenum and titanium sulfide compounds. Annealing Ti/MoS2 sequentially at 100, 300, and 600 °C for 30 min in UHV results in a gradual increase in the reaction products as measured by XPS. Accordingly, STEM reveals the formation of a new ordered phase and a Mo-rich layer at the interface following heating. Due to the high degree of reactivity, the Ti/MoS2 interface is not thermally stable even at a transistor operating temperature of 100 °C, while post-deposition annealing further enhances the interfacial reactions. These findings have important consequences for electrical transport properties, highlighting the importance of interface chemistry in the metal contact design and fabrication.

5.
ACS Nano ; 13(7): 8035-8046, 2019 Jul 23.
Artigo em Inglês | MEDLINE | ID: mdl-31247141

RESUMO

The coexistence of metallic and semiconducting polymorphs in transition-metal dichalcogenides (TMDCs) can be utilized to solve the large contact resistance issue in TMDC-based field effect transistors (FETs). A semiconducting hexagonal (2H) molybdenum ditelluride (MoTe2) phase, metallic monoclinic (1T') MoTe2 phase, and their lateral homojunctions can be selectively synthesized in situ by chemical vapor deposition due to the small free energy difference between the two phases. Here, we have investigated, in detail, the structural and electrical properties of in situ-grown lateral 2H/1T' MoTe2 homojunctions grown using flux-controlled phase engineering. Using atomic-resolution plan-view and cross-sectional transmission electron microscopy analyses, we show that the round regions of near-single-crystalline 2H-MoTe2 grow out of a polycrystalline 1T'-MoTe2 matrix. We further demonstrate the operation of MoTe2 FETs made on these in situ-grown lateral homojunctions with 1T' contacts. The use of a 1T' phase as electrodes in MoTe2 FETs effectively improves the device performance by substantially decreasing the contact resistance. The contact resistance of 1T' electrodes extracted from transfer length method measurements is 470 ± 30 Ω·µm. Temperature- and gate-voltage-dependent transport characteristics reveal a flat-band barrier height of ∼30 ± 10 meV at the lateral 2H/1T' interface that is several times smaller and shows a stronger gate modulation, compared to the metal/2H Schottky barrier height. The information learned from this analysis will be critical to understanding the properties of MoTe2 homojunction FETs for use in memory and logic circuity applications.

6.
ACS Nano ; 13(1): 377-385, 2019 Jan 22.
Artigo em Inglês | MEDLINE | ID: mdl-30563322

RESUMO

Band-to-band tunneling field-effect transistors (TFETs) have emerged as promising candidates for low-power integration circuits beyond conventional metal-oxide-semiconductor field-effect transistors (MOSFETs) and have been demonstrated to overcome the thermionic limit, which results intrinsically in sub-threshold swings of at least 60 mV/dec at room temperature. Here, we demonstrate complementary TFETs based on few-layer black phosphorus, in which multiple top gates create electrostatic doping in the source and drain regions. By electrically tuning the doping types and levels in the source and drain regions, the device can be reconfigured to allow for TFET or MOSFET operation and can be tuned to be n-type or p-type. Owing to the proper choice of materials and careful engineering of device structures, record-high current densities have been achieved in 2D TFETs. Full-band atomistic quantum transport simulations of the fabricated devices agree quantitatively with the current-voltage measurements, which gives credibility to the promising simulation results of ultrascaled phosphorene TFETs. Using atomistic simulations, we project substantial improvements in the performance of the fabricated TFETs when channel thicknesses and oxide thicknesses are scaled down.

7.
Nat Mater ; 18(1): 55-61, 2019 01.
Artigo em Inglês | MEDLINE | ID: mdl-30542093

RESUMO

Transition metal dichalcogenides have attracted attention as potential building blocks for various electronic applications due to their atomically thin nature and polymorphism. Here, we report an electric-field-induced structural transition from a 2H semiconducting to a distorted transient structure (2Hd) and orthorhombic Td conducting phase in vertical 2H-MoTe2- and Mo1-xWxTe2-based resistive random access memory (RRAM) devices. RRAM programming voltages are tunable by the transition metal dichalcogenide thickness and show a distinctive trend of requiring lower electric fields for Mo1-xWxTe2 alloys versus MoTe2 compounds. Devices showed reproducible resistive switching within 10 ns between a high resistive state and a low resistive state. Moreover, using an Al2O3/MoTe2 stack, On/off current ratios of 106 with programming currents lower than 1 µA were achieved in a selectorless RRAM architecture. The sum of these findings demonstrates that controlled electrical state switching in two-dimensional materials is achievable and highlights the potential of transition metal dichalcogenides for memory applications.

8.
Sci Rep ; 8(1): 8668, 2018 Jun 06.
Artigo em Inglês | MEDLINE | ID: mdl-29875367

RESUMO

Environmental and thermal stability of two-dimensional (2D) transition metal dichalcogenides (TMDs) remains a fundamental challenge towards enabling robust electronic devices. Few-layer 2H-MoTe2 with an amorphous boron nitride (a-BN) covering layer was synthesized as a channel for back-gated field effect transistors (FET) and compared to uncovered MoTe2. A systematic approach was taken to understand the effects of heat treatment in air on the performance of FET devices. Atmospheric oxygen was shown to negatively affect uncoated MoTe2 devices while BN-covered FETs showed considerably enhanced chemical and electronic characteristic stability. Uncapped MoTe2 FET devices, which were heated in air for one minute, showed a polarity switch from n- to p-type at 150 °C, while BN-MoTe2 devices switched only after 200 °C of heat treatment. Time-dependent experiments at 100 °C showed that uncapped MoTe2 samples exhibited the polarity switch after 15 min of heat treatment while the BN-capped device maintained its n-type conductivity for the maximum 60 min duration of the experiment. X-ray photoelectron spectroscopy (XPS) analysis suggests that oxygen incorporation into MoTe2 was the primary doping mechanism for the polarity switch. This work demonstrates the effectiveness of an a-BN capping layer in preserving few-layer MoTe2 material quality and controlling its conductivity type at elevated temperatures in an atmospheric environment.

9.
Nano Lett ; 18(4): 2485-2491, 2018 04 11.
Artigo em Inglês | MEDLINE | ID: mdl-29561623

RESUMO

Ultrathin transition metal dichalcogenides (TMDCs) have recently been extensively investigated to understand their electronic and optical properties. Here we study ultrathin Mo0.91W0.09Te2, a semiconducting alloy of MoTe2, using Raman, photoluminescence (PL), and optical absorption spectroscopy. Mo0.91W0.09Te2 transitions from an indirect to a direct optical band gap in the limit of monolayer thickness, exhibiting an optical gap of 1.10 eV, very close to its MoTe2 counterpart. We apply tensile strain, for the first time, to monolayer MoTe2 and Mo0.91W0.09Te2 to tune the band structure of these materials; we observe that their optical band gaps decrease by 70 meV at 2.3% uniaxial strain. The spectral widths of the PL peaks decrease with increasing strain, which we attribute to weaker exciton-phonon intervalley scattering. Strained MoTe2 and Mo0.91W0.09Te2 extend the range of band gaps of TMDC monolayers further into the near-infrared, an important attribute for potential applications in optoelectronics.

10.
Phys Rev B ; 982018.
Artigo em Inglês | MEDLINE | ID: mdl-30984898

RESUMO

Transition-metal dichalcogenides (TMDCs) have offered experimental access to quantum confinement in one dimension. In recent years, metallic TMDCs like NbSe2 have taken center stage with many of them exhibiting interesting temperature-dependent properties such as charge density waves and superconductivity. In this paper, we perform a comprehensive optical analysis of NbSe2 by utilizing Raman spectroscopy, differential reflectance contrast, and spectroscopic ellipsometry. These analyses, when coupled with Kramers-Kronig analysis, allow us to extract the dielectric functions of bulk and atomically thin NbSe2 and relate them to the resonant behavior of the Raman spectra.

11.
Phys Rev Mater ; 1(2)2017 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-28890947

RESUMO

We report a rectangular charge density wave (CDW) phase in strained 1T-VSe2 thin films synthesized by molecular beam epitaxy on c-sapphire substrates. The observed CDW structure exhibits an unconventional rectangular 4a×√3a periodicity, as opposed to the previously reported hexagonal 4a×4a structure in bulk crystals and exfoliated thin layered samples. Tunneling spectroscopy shows a strong modulation of the local density of states of the same 4a×√3a CDW periodicity and an energy gap of 2ΔCDW = (9.1 ± 0.1) meV. The CDW energy gap evolves into a full gap at temperatures below 500 mK, indicating a transition to an insulating phase at ultra-low temperatures. First-principles calculations confirm the stability of both 4a×4a and 4a×√3a structures arising from soft modes in the phonon dispersion. The unconventional structure becomes preferred in the presence of strain, in agreement with experimental findings.

12.
Nanotechnology ; 28(43): 435601, 2017 Oct 27.
Artigo em Inglês | MEDLINE | ID: mdl-28854152

RESUMO

Novel nanofluidic chemical cells based on self-assembled solid-state SiO2 nanotubes on silicon-on-insulator (SOI) substrate have been successfully fabricated and characterized. The vertical SiO2 nanotubes with a smooth cavity are built from Si nanowires which were epitaxially grown on the SOI substrate. The nanotubes have rigid, dry-oxidized SiO2 walls with precisely controlled nanotube inner diameter, which is very attractive for chemical-/bio-sensing applications. No dispersion/aligning procedures were involved in the nanotube fabrication and integration by using this technology, enabling a clean and smooth chemical cell. Such a robust and well-controlled nanotube is an excellent case of developing functional nanomaterials by leveraging the strength of top-down lithography and the unique advantage of bottom-up growth. These solid, smooth, clean SiO2 nanotubes and nanofluidic devices are very encouraging and attractive in future bio-medical applications, such as single molecule sensing and DNA sequencing.

13.
Chem Mater ; 29(15): 6279-6288, 2017 Aug 08.
Artigo em Inglês | MEDLINE | ID: mdl-29545674

RESUMO

High volume manufacturing of devices based on transition metal dichalcogenide (TMD) ultra-thin films will require deposition techniques that are capable of reproducible wafer-scale growth with monolayer control. To date, TMD growth efforts have largely relied upon sublimation and transport of solid precursors with minimal control over vapor phase flux and gas-phase chemistry, which are critical for scaling up laboratory processes to manufacturing settings. To address these issues, we report a new pulsed metalorganic chemical vapor deposition (MOCVD) route for MoS2 film growth in a research-grade single-wafer reactor. Using bis(tert-butylimido)-bis(dimethylamido)molybdenum and diethyl disulfide we deposit MoS2 films from ≈ 1 nm to ≈ 25 nm in thickness on SiO2/Si substrates. We show that layered 2H-MoS2 can be produced at comparatively low reaction temperatures of 591 °C at short deposition times, approximately 90 s for few-layer films. In addition to the growth studies performed on SiO2/Si, films with wafer-level uniformity are demonstrated on 50 mm quartz wafers. Process chemistry and impurity incorporation from precursors are also discussed. This low-temperature and fast process highlights the opportunities presented by metalorganic reagents in the controlled synthesis of TMDs.

14.
Adv Electron Mater ; 2(9)2016 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-27840807

RESUMO

The need for low-cost high-performance broadband photon detection with sensitivity in the near infrared (NIR) has driven interest in new materials that combine high absorption with traditional electronic infrastructure (CMOS) compatibility. Here, we demonstrate a facile, low-cost and scalable, catalyst-free one-step solution-processed approach to grow one-dimensional Sb2Se3 nanostructures directly on flexible substrates for high-performance NIR photodetectors. Structural characterization and compositional analyses reveal high-quality single-crystalline material with orthorhombic crystal structure and a near-stoichiometric Sb/Se atomic ratio. We measure a direct band gap of 1.12 eV, which is consistent with predictions from theoretical simulations, indicating strong NIR potential. The fabricated metal-semiconductor-metal photodetectors exhibit fast response (on the order of milliseconds) and high performance (responsivity ~ 0.27 A/W) as well as excellent mechanical flexibility and durability. The results demonstrate the potential of molecular-ink-based Sb2Se3 nanostructures for flexible electronic and broadband optoelectronic device applications.

15.
ACS Nano ; 10(10): 9626-9636, 2016 Oct 25.
Artigo em Inglês | MEDLINE | ID: mdl-27704774

RESUMO

We study the crystal symmetry of few-layer 1T' MoTe2 using the polarization dependence of the second harmonic generation (SHG) and Raman scattering. Bulk 1T' MoTe2 is known to be inversion symmetric; however, we find that the inversion symmetry is broken for finite crystals with even numbers of layers, resulting in strong SHG comparable to other transition-metal dichalcogenides. Group theory analysis of the polarization dependence of the Raman signals allows for the definitive assignment of all the Raman modes in 1T' MoTe2 and clears up a discrepancy in the literature. The Raman results were also compared with density functional theory simulations and are in excellent agreement with the layer-dependent variations of the Raman modes. The experimental measurements also determine the relationship between the crystal axes and the polarization dependence of the SHG and Raman scattering, which now allows the anisotropy of polarized SHG or Raman signal to independently determine the crystal orientation.

16.
Nano Lett ; 16(10): 6064-6069, 2016 10 12.
Artigo em Inglês | MEDLINE | ID: mdl-27603879

RESUMO

Impurity doping in two-dimensional (2D) materials can provide a route to tuning electronic properties, so it is important to be able to determine the distribution of dopant atoms within and between layers. Here we report the tomographic mapping of dopants in layered 2D materials with atomic sensitivity and subnanometer spatial resolution using atom probe tomography (APT). APT analysis shows that Ag dopes both Bi2Se3 and PbSe layers in (PbSe)5(Bi2Se3)3, and correlations in the position of Ag atoms suggest a pairing across neighboring Bi2Se3 and PbSe layers. Density functional theory (DFT) calculations confirm the favorability of substitutional doping for both Pb and Bi and provide insights into the observed spatial correlations in dopant locations.

17.
Nanotechnology ; 27(31): 31LT02, 2016 Aug 05.
Artigo em Inglês | MEDLINE | ID: mdl-27325151

RESUMO

In this letter, fracture strengths σ f of native and oxidized silicon nanowires (SiNWs) were determined via atomic force microscopy bending experiments and nonlinear finite element analysis. In the native SiNWs, σ f in the Si was comparable to the theoretical strength of Si〈111〉, ≈22 GPa. In the oxidized SiNWs, σ f in the SiO2 was comparable to the theoretical strength of SiO2, ≈6 to 12 GPa. The results indicate a change in the failure mechanism between native SiNWs, in which fracture originated via inter-atomic bond breaking or atomic-scale defects in the Si, and oxidized SiNWs, in which fracture initiated from surface roughness or nano-scale defects in the SiO2.

18.
ACS Appl Mater Interfaces ; 8(18): 11390-5, 2016 05 11.
Artigo em Inglês | MEDLINE | ID: mdl-27100021

RESUMO

Transition metal dichalcogenides (TMDCs) have been known for decades to have unique properties and recently attracted broad attention for their two-dimensional (2D) characteristics. NbSe2 is a metallic TMDC that has been studied for its charge density wave transition behavior and superconductivity but is still largely unexplored for its potential use in engineered devices with applications in areas such as electronics, optics, and batteries. Thus, we successfully demonstrate and present evidence of lithium intercalation in NbSe2 as a technique capable of modifying the material properties of NbSe2 for further study. We demonstrate successful intercalation of Li ions into NbSe2 and confirm this result through X-ray diffraction, noting a unit cell size increase from 12.57 to 13.57 Å in the c lattice parameter of the NbSe2 after intercalation. We also fabricate planar half-cell electrochemical devices using ultrathin NbSe2 from platelets to observe evidence of Li-ion intercalation through an increase in the optical transmittance of the material in the visible range. Using 550 nm wavelength light, we observed an increase in optical transmittance of 26% during electrochemical intercalation.

19.
ACS Nano ; 10(3): 3580-8, 2016 Mar 22.
Artigo em Inglês | MEDLINE | ID: mdl-26866442

RESUMO

When designing semiconductor heterostructures, it is expected that epitaxial alignment will facilitate low-defect interfaces and efficient vertical transport. Here, we report lattice-matched epitaxial growth of molybdenum disulfide (MoS2) directly on gallium nitride (GaN), resulting in high-quality, unstrained, single-layer MoS2 with strict registry to the GaN lattice. These results present a promising path toward the implementation of high-performance electronic devices based on 2D/3D vertical heterostructures, where each of the 3D and 2D semiconductors is both a template for subsequent epitaxial growth and an active component of the device. The MoS2 monolayer triangles average 1 µm along each side, with monolayer blankets (merged triangles) exhibiting properties similar to that of single-crystal MoS2 sheets. Photoluminescence, Raman, atomic force microscopy, and X-ray photoelectron spectroscopy analyses identified monolayer MoS2 with a prominent 20-fold enhancement of photoluminescence in the center regions of larger triangles. The MoS2/GaN structures are shown to electrically conduct in the out-of-plane direction, confirming the potential of directly synthesized 2D/3D semiconductor heterostructures for vertical current flow. Finally, we estimate a MoS2/GaN contact resistivity to be less than 4 Ω·cm(2) and current spreading in the MoS2 monolayer of approximately 1 µm in diameter.

20.
Nano Lett ; 15(2): 1122-7, 2015 Feb 11.
Artigo em Inglês | MEDLINE | ID: mdl-25625509

RESUMO

Optimizing new generations of two-dimensional devices based on van der Waals materials will require techniques capable of measuring variations in electronic properties in situ and with nanometer spatial resolution. We perform scanning microwave microscopy (SMM) imaging of single layers of MoS2 and n- and p-doped WSe2. By controlling the sample charge carrier concentration through the applied tip bias, we are able to reversibly control and optimize the SMM contrast to image variations in electronic structure and the localized effects of surface contaminants. By further performing tip bias-dependent point spectroscopy together with finite element simulations, we distinguish the effects of the quantum capacitance and determine the local dominant charge carrier species and dopant concentration. These results underscore the capability of SMM for the study of 2D materials to image, identify, and study electronic defects.

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