Session I6: Materials VII: Nanostructures II

Probing thermal transport at the nanoscale: the collectively-diffusive regime

How is thermal transport modified by the geometry of nanostructured systems? Our research seeks to answer this question and gain a deeper understanding of phonon dynamics, the main heat carriers in semiconductor and dielectric materials. Today, advances in nanofabrication make it possible to pattern nanostructures with dimensions \textless 10nm, much smaller than the phonon mean free path in many materials. At these length scales, Fourier's law of heat diffusion cannot accurately describe the thermal dynamics of even simple systems, and experimental results are needed to guide theoretical efforts. In order to study nanoscale thermal transport at its characteristic length- and time-scales, we take advantage of the tabletop high harmonic generation (HHG) process to generate ultrashort, coherent, extreme ultraviolet (EUV) probe light. The short wavelength of EUV beams is sensitive to picometer thermal deformations of the surface; and the femtosecond duration of HHG pulses is fast enough to capture sub-picosecond thermal dynamics of nanostructured systems. We follow the heat dissipation away from periodic arrays of metallic nanowires (20-1000nm wide) on top of both silicon and sapphire substrates. By comparing the thermal relaxation of nanowire arrays of the same linewidth but different periodicities, we confirm the predictions of the recently uncovered \textit{collectively-diffusive} regime: closely-spaced nanowires cool faster than widely-spaced ones.   

Supported Single Atoms Mediate Catalytic Reactions

Isolated single metal atoms dispersed on high-surface-area supports have recently demonstrated remarkable activity and selectivity for a variety of catalytic reactions. The interaction of the individual metal atoms with the support surface modifies the surface electronic structure of the metal-support ensembles and thus tunes the binding strength of the reactant molecules. Such an approach to engineering the surface electronic structure of high-surface-area support materials can be effectively utilized for developing new and better catalysts. The realization of atomically dispersed, supported metal catalysts, especially noble metal catalysts, is not only of fundamental interest but also opens new routes to significantly reduce the cost of practical catalysts for a plethora of important chemical transformations. The challenges in developing practical single atom catalysts (SACs) include a) robust synthesis of SACs with high levels of metal loading and b) stabilization of isolated single atoms during catalysis. Anchoring of the isolated single metal atoms onto the surfaces of the supports is therefore critical. Aberration-corrected electron microscopy techniques prove to be invaluable for unambiguously identifying the location of the isolated single metal atoms and the surface atomic structure of the supports. The fundamental physics of the electron transfer processes between the deposited single metal atoms and the support surfaces is still not fully understood.   

Novel cold-wall CVD synthesis of highly uniform MoS$_{\mathrm{2}}$ thin films

We present results from a novel cold-wall chemical vapor deposition method for growing MoS$_{\mathrm{2}}$. This method affords independent control over all deposition parameters. Ar carrier gas flow rate and pressure, substrate temperature and the temperatures of the individual solid-source precursors can all be independently varied during growth onto 100 nm-thick SiO$_{\mathrm{2}}$ films on Si substrates. Individually optimizing each deposition parameter enables the formation of islanded, single-layer MoS$_{\mathrm{2}}$ films. At the optimal growth parameters, we were able to repeatably grow samples with a Mo coverage that corresponds to 0.3 \textpm 0.03 ML of MoS$_{\mathrm{2}}$. Mo coverage, feature size and feature density on these samples exhibited uniformity over the central 32 mm$^{\mathrm{2}}$ of the sample.   

Tuning Magnetism of Zirconium Disulfide Nanoribbons by Strain

Monolayer transition metal dichalcogenides have recently attracted considerable attention due to their unusual physical properties and potential applications in nanoscale electronic devices. We carried out \textit{ab initio} density functional calculations to study the electronic and magnetic properties of strained ZrS$_2$ nanoribbons. Our calculations demonstrated that ZrS$_2$ nanoribbons without edge passivation were non-magnetic. In contrast, we found that ZrS$_2$ nanoribbons passivated with hydrogen atoms could switch between the regimes of magnetic and non-magnetic behaviour. Our study showed that edge-passivated armchair ZrS$_2$ nanoribbons were magnetic under applied strain up to 6\%,whereas zigzag ZrS$_2$ nanoribbons were magnetic under applied strain between 7\% and 12\%. The results of our calculations suggested the possibility of tuning the magnetism of ZrS$_2$ nanoribbons by changing the applied strain.   

A study of graphene nucleation density on supported thin solid copper films

We report the controlled growth of graphene on 1.25~$\mu $m Cu films sputter-deposited on tungsten employing a cold-wall CVD method. The supported Cu films were resistively heated to 1000$^{o}$C at a chamber pressure of 700 Torrs. Precursor flow rates of 7, 1000 and 10,000 sccm for CH$_{4}$,~H$_{2}$,~and Ar were employed. For the range of growth times investigated, 7-12 mins, the graphene films comprised isolated, single layer, hexagonal nuclei as determined by scanning electron microscope and Raman spectroscopy. The nucleation density vs. time profile comprises an initial regime of the first 6 mins of growth during which no graphene is observed indicating undersaturated C concentration on the Cu surface. Supersaturation is reached near 7 mins, when the first graphene nuclei are observed with a density of 3.5X10$^{5}$/cm$^{2}$. The graphene nucleus density rises during 8-10 mins and reaches its saturation value of 1X10$^{6}$/cm$^{2}$. A well-defined plateau was evident for the next 3 mins during which the average diameter of the graphene flakes increased from 3.6 to 4.6~$\mu $m. Finally, continued growth of the isolated nuclei leads to impingement and eventual formation of a complete graphene layer.   

\textbf{\textit{In Situ}}\textbf{ Resistivity of Endotaxial FeSi}$_{\mathrm{\mathbf{2}}}$\textbf{ Nanowires on Si(110)}

We present \textit{in situ} Ultra-High Vacuum (UHV) measurements of the resistivity $\rho $ of self-assembled endotaxial FeSi$_{\mathrm{2}}$ nanowires (NWs) on Si(110) using a variable-spacing two-point method with a moveable Scanning Tunneling Microscope (STM) tip and fixed contact pad. The resistivity at room temperature was found to be nearly constant down to NW width W $=$ 4 nm, but rose sharply to nearly double the bulk value at W $=$ 3nm. These data are not well-fit by a simple Fuch-Sondheimer model for boundary scattering, suggesting that other factors, possibly quantum effects, may be significant at the smallest dimensions. For a NW width of 4 nm, partial oxidation increased $\rho $ by approximately 50{\%}, while cooling from 300K to 150K decreased $\rho $ by approximately 10{\%}. The relative insensitivity of $\rho $ to NW size or oxidation or cooling is attributed to a high concentration of vacancies in the FeSi$_{\mathrm{2}}$ structure, with a correspondingly short length for inelastic electron scattering, which obscures boundary scattering except in the smallest NWs. It is remarkable that the vacancy concentration persists in very small structures.