Transition metal dichalcogenides (TMDs) and 2-dim materials beyond graphene have shown excellent potential for future electronics. Controlling the heat flow across a hetero-structure will be crucial to developing high-speed electronic devices based on 2-dim materials. We have recently shown that the thermal expansion coefficient (TEC) dramatically increases in 2-dim materials when the thickness of the material shrinks from bulk to a few monolayers. Therefore, the TEC mismatch of 2-dim materials becomes an additional concern in designing electronic nano-devices. More specifically, we need to develop methods that enable us to control and tailor the TEC of TMDs through alloying or defect engineering.
In this contribution, I will employ transition metal alloying in TMDs to tune the TEC of monolayer Mo1-xWxS2 and study the interplay between thermal expansion and local defects using a combination of the scanning transmission electron microscope (STEM), electron energy loss spectroscopy (EELS) and first-principles DFT calculations. More specifically, we will measure the thermal expansion coefficient based on the plasmon energy shift as a function of temperature and combine this with first-principles modeling of the low-loss EELS signals. Using DFT calculations in the random phase approximation (RPA) we model the the plasmon peak shift as a function of lattice expansion. Combining the experimental and modeling data, we can now predict the TEC for WSe2.
Using this approach, we have determined the TEC of monolayer MoS2 and WS2 and found a significant mismatch between the two materials. To explore the influence of alloy engineering on the TEC, free-standing Mo0.7W0.3S2 2-dim materials are prepared. Finally, I will compare the TEC of alloyed Mo0.7W0.3S2 monolayer with that of MoS2/WS2 lateral heterointerfaces and explore the effects of strain or point defects on the local TEC using a combination of STEM imaging, EEL spectroscopy and DFT modeling.
Given the large thermal activation energy of acceptors in high %Al AlGaN, a new approach is needed to control p-type
conductivity in this material. One promising alternative to using impurity doping with thermal activation is using the
intrinsic characteristics of the III-nitrides to activate dopants with polarization-induced charge in graded heterostructures.
In this work polarization-induced activation of dopants is used in graded AlGaN nanowires grown by plasma-assisted
molecular beam epitaxy to form ultraviolet light-emitting diodes. Electrical and optical characterization is provided,
showing clear diode behavior and electroluminescent emission at 336nm. Variable temperature electrical measurements
show little change in device performance at cryogenic temperatures, proving that dopant ionization is polarizationinduced
rather than thermally activated.
In this paper, we will describe the experimental processes involved in analytical atomic-resolution scanning transmission electron microscopy (STEM) of supported nano-scale systems. We show that the combination of high-resolution Z-contrast imaging and electron energy loss spectroscopy (EELS) provides an analytical tool with unprecedented chemical and spatial sensitivity that is vital for studying interfaces in heterogeneous catalyst systems. We apply the described methods to study two example heterogeneous catalyst systems: Pt/SiO2, and Cu/Al2O3. In particular, the presence of a few monolayers of platinum oxide in Pt/SiO2 can be clearly seen, and changes in the chemistry of the SiO2 support within ~1 nm of the metal-oxide interface can be characterized as a function of the catalyst preparation conditions. The Cu/Al2O3, reduced at various temperatures, exhibits an increasing oxidation of the Cu-particles upon higher temperature reduction.
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