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Dhall, Rohan

Publications and source records attributed to Dhall, Rohan.

31 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Bulk direct gap MoS2 by plasma induced layer decoupling

Bulk direct transition metal dichalcogenide (TMDC) may have an increased interlayer separation of at least 0.5, 1, or 3 angstroms more than its bulk value. The TMDC may be a bulk direct band gap molybdenum disulfide (MoS2) or a bulk direct band gap tungsten diselenide (WSe2). Oxygen may be between the interlayers. A device may include the TMDC, such as an optoelectronic device, such as an LED, solid state laser, a photodetector, a solar cell, a FET, a thermoelectric generator, or a thermoelectric cooler. A method of making bulk direct transition metal dichalcogenide (TMDC) with increased interlayer separation may include exposing bulk direct TMDC to a remote (aka downstream) oxygen plasma. The plasma exposure may cause an increase in the photoluminescence efficiency of the TMDC, more charge neutral doping, or longer photo-excited carrier lifetimes, as compared to the TMDC without the plasma exposure.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Measuring nanoscale thermal gradients in suspended MoS2 with STEM-EELS

Transition metal dichalcogenides such as molybdenum disulfide (MoS2) may see service in the heart of next-generation nanoelectronic devices, where highly localized power dissipation can produce nontrivial temperature gradients over nanometer-scale distances. Here, we demonstrate that MoS2 is a promising target for plasmon energy expansion thermometry (PEET), a high-spatial resolution temperature mapping technique employed in a scanning transmission electron microscope (STEM) equipped with electron energy loss spectroscopy (EELS). We first use a calibrated, commercial MEMS-style TEM sample heater chip to measure the temperature dependence of the MoS2 bulk plasmon. We corroborate the chip's temperature calibration with Raman thermometry and determine the bulk thermal expansion coefficient (TEC) of MoS2 in the temperature range of 300–1100 K. Applying this TEC value to PEET measurements on a suspended MoS2 flake, we map 70–90 K/μm temperature gradients with a submicrometer spatial resolution.

Shen, Lang (ORCID:0000000297554821)↗
DOE OSTI2018

Defect-Induced Photoluminescence Enhancement and Corresponding Transport Degradation in Individual Suspended Carbon Nanotubes

The utilization of defects in carbon nanotubes to improve their photoluminescence efficiency has become a widespread study towards the realization of efficient light emitting devices. Here, we report a detailed comparison of defects in nanotubes (quantified by Raman spectroscopy) and photoluminescence (PL) intensity of individual suspended carbon nanotubes (CNTs). We have also the evaluated the impact of these defects on the electron/hole transport in the nanotubes, which is crucial for the ultimate realization of optoelectronic devices. We find that brightly luminescent nanotubes exhibit a pronounced D-band in their Raman spectra, and vice versa, dimly luminescent nanotubes exhibit almost no D-band. Here, defects are advantageous for light emission by trapping excitons, which extends their lifetimes. We quantify this behavior by plotting the PL intensity as a function of the I D /I G band Raman intensity ratio, which exhibits a Lorentz distribution peaked at I D /I G =0.17. For CNTs with a I D /I G ratio >0.25, the PL intensity decreases, indicating that, above some critical density, non-radiative recombination at defect sites dominates over the advantages of exciton trapping. In an attempt to fabricate optoelectronic devices based on these brightly luminescent CNTs, we transferred these suspended CNTs to platinum electrodes and found that the brightly photoluminescent nanotubes exhibit nearly infinite resistance due to these defects while those without bright photoluminescence exhibit finite resistance. These findings indicate a potential limitation in the use of brightly luminescent CNTs for optoelectronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Probing collective oscillation of d -orbital electrons at the nanoscale

Here, we demonstrate that high energy electrons can be used to explore the collective oscillation of $s$, $p$, and $d$ orbital electrons at the nanometer length scale. Using epitaxial AlGaN/AlN quantum wells as a test system, we observe the emergence of additional features in the loss spectrum with the increasing Ga content. We report a comparison of the observed spectra with ab-initio theory reveals that the origin of these spectral features lies in excitations of 3$d$-electrons contributed by Ga. We find that these modes differ in energy from the valence electron plasmons in Al 1-x Ga x N due to the different polarizabilities of the d electrons. Finally, we study the dependence of observed spectral features on the Ga content, lending insights into the origin of these spectral features, and their coupling with electron-hole excitations.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Charge neutral MoS 2 field effect transistors through oxygen plasma treatment

Lithographically fabricated MoS 2 field effect transistors suffer from several critical imperfections, including low sub-threshold swings, large turn-on gate voltages (V T ), and wide device-to-device variability. The large magnitude and variability of V T stems from unclean interfaces, trapped charges in the underlying substrate, and sulfur vacancies created during the mechanical exfoliation process. In this work, we demonstrate a simple and reliable oxygen plasma treatment, which mitigates the effects of unintentional doping created by surface defect sites, such as S vacancies, and surface contamination. This plasma treatment restores charge neutrality to the MoS 2 and shifts the threshold turn-on voltage towards 0 V. Out of the 10 devices measured, all exhibit a shift of the FET turn-on voltage from an average of -18 V to -2 V. The oxygen plasma treatment passivates these defects, which reduces surface scattering, causing increased mobility and improved subthreshold swing. For as-prepared devices with low mobilities (~0.01 cm 2 /V s), we observe up to a 190-fold increase in mobility after exposure to the oxygen plasma. Perhaps the most important aspect of this oxygen plasma treatment is that it reduces the device-to-device variability, which is a crucial factor in realizing any practical application of these devices.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Enhanced photoluminescence in air-suspended carbon nanotubes by oxygen doping

We report photoluminescence (PL) imaging and spectroscopy of air-suspended carbon nanotubes (CNTs) before and after exposure to a brief (20 s) UV/ozone treatment. These spectra show enhanced PL intensities in 10 out of 11 nanotubes that were measured, by as much as 5-fold. This enhancement in the luminescence efficiency is caused by oxygen defects which trap excitons. We also observe an average 3-fold increase in the D-band Raman intensity further indicating the creation of defects. Previous demonstrations of oxygen doping have been carried out on surfactantcoated carbon nanotubes dissolved in solution, thus requiring substantial longer ozone/UV exposure times (~15 h). Here, the ozone treatment is more efficient because of the surface exposure of the air-suspended CNTs. In addition to enhanced PL intensities, we observe narrowing of the emission linewidth by 3–10 nm. Finally, this ability to control and engineer defects in CNTs is important for realizing several optoelectronic applications such as light-emitting diodes and single photon sources.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Imaging interfacial electrical transport in graphene–MoS{sub 2} heterostructures with electron-beam-induced-currents

Heterostructure devices with specific and extraordinary properties can be fabricated by stacking two-dimensional crystals. Cleanliness at the inter-crystal interfaces within a heterostructure is crucial for maximizing device performance. However, because these interfaces are buried, characterizing their impact on device function is challenging. Here, we show that electron-beam induced current (EBIC) mapping can be used to image interfacial contamination and to characterize the quality of buried heterostructure interfaces with nanometer-scale spatial resolution. We applied EBIC and photocurrent imaging to map photo-sensitive graphene-MoS{sub 2} heterostructures. The EBIC maps, together with concurrently acquired scanning transmission electron microscopy images, reveal how a device's photocurrent collection efficiency is adversely affected by nanoscale debris invisible to optical-resolution photocurrent mapping.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Nanoscale temperature mapping in operating microelectronic devices

We report that modern microelectronic devices have nanoscale features that dissipate power nonuniformly, but fundamental physical limits frustrate efforts to detect the resulting temperature gradients. Contact thermometers disturb the temperature of a small system, while radiation thermometers struggle to beat the diffraction limit. Exploiting the same physics as Fahrenheit’s glass-bulb thermometer, we mapped the thermal expansion of Joule-heated, 80-nanometer-thick aluminum wires by precisely measuring changes in density. With a scanning transmission electron microscope (STEM) and electron energy loss spectroscopy (EELS), we quantified the local density via the energy of aluminum’s bulk plasmon. Rescaling density to temperature yields maps with a statistical precision of 3 kelvin/hertz ₋1/2 , an accuracy of 10%, and nanometer-scale resolution. Lastly, many common metals and semiconductors have sufficiently sharp plasmon resonances to serve as their own thermometers.

47 OTHER INSTRUMENTATION↗