Tianmin Wu

Ultrafast formation of interlayer hot excitons in atomically thin MoS2/WS2 heterostructures

Van der Waals heterostructures composed of two-dimensional transition-metal dichalcogenides layers have recently emerged as a new family of materials, with great potential for atomically thin opto-electronic and photovoltaic applications. It is puzzling, however, that the photocurrent is yielded so efficiently in these structures, despite the apparent momentum mismatch between the intralayer/interlayer excitons during the charge transfer, as well as the tightly bound nature of the excitons in 2D geometry. Using the energy-state-resolved ultrafast visible/infrared microspectroscopy, we herein obtain unambiguous experimental evidence of the charge transfer intermediate state with excess energy, during the transition from an intralayer exciton to an interlayer exciton at the interface of a WS2/MoS2 heterostructure, and free carriers moving across the interface much faster than recombining into the intralayer excitons. The observations therefore explain how the remarkable charge transfer rate and photocurrent generation are achieved even with the aforementioned momentum mismatch and excitonic localization in 2D heterostructures and devices.

Molecular mechanism of water reorientational slowing down in concentrated ionic solutions

Significance The dynamics of water molecules surrounding the hydrated ions affects many natural phenomena including protein processes and charge transfer in the aqueous rechargeable ion batteries. In the concentrated solutions, a long-standing puzzle is that all electrolytes retard water rotation regardless of whether they weaken or strengthen the water hydrogen-bonding network. We investigate this issue theoretically and find the deceleration to be largely due to the coupling of the slow, collective component of water rotation with the motion of sizable ion clusters in the concentrated solutions. This finding is at variance with the intuitive expectation that the deceleration is caused by the change in fast, single-molecular water hydrogen bond switching adjacent to the ions. Water dynamics in concentrated ionic solutions plays an important role in a number of material and energy conversion processes such as the charge transfer at the electrolyte–electrode interface in aqueous rechargeable ion batteries. One long-standing puzzle is that all electrolytes, regardless of their “structure-making/breaking” nature, make water rotate slower at high concentrations. To understand this effect, we present a theoretical simulation study of the reorientational motion of water molecules in different ionic solutions. Using an extended Ivanov model, water rotation is decomposed into contributions from large-amplitude angular jumps and a slower frame motion which was studied in a coarse-grained manner. Bearing a certain resemblance to water rotation near large biological molecules, the general deceleration is found to be largely due to the coupling of the slow, collective component of water rotation with the motion of large hydrated ion clusters ubiquitously existing in the concentrated ionic solutions. This finding is at variance with the intuitive expectation that the slowing down is caused by the change in fast, single-molecular water hydrogen bond switching adjacent to the ions.

Low frequency 2D Raman-THz spectroscopy of ionic solution: A simulation study

The 2D Raman-THz spectrum of the MgCl2 solution was simulated using the molecular dynamics simulation and the stability matrix method and compared with that of the pure water. The 2D Raman-THz signal provides more information on the ion effects on the collective water motion than the conventional 1D signal. The presence of MgCl2 suppresses the cross peak of water between the hydrogen bond bending and the other intermolecular vibrational mode, which clearly illustrates that the water hydrogen bending motion is affected by the confining effect of the ions. Our theoretical work thus demonstrates that the 2D Raman-THz technique can become a valuable nonlinear vibrational probe for the molecular dynamics in the ionic solutions.

Discriminating trpzip2 and trpzip4 peptides' folding landscape using the two-dimensional infrared spectroscopy: a simulation study.

We analyzed, based on the theoretical spectroscopic modeling, how the differences in the folding landscapes of two β-hairpin peptides trpzip2 and trpzip4 are reflected in their thermal unfolding infrared measurements. The isotope-edited equilibrium FTIR and two dimensional infrared spectra of the two peptides were calculated, using the nonlinear exciton propagation method, at a series of temperatures. The spectra calculations were based on the configuration distributions generated using the GB(OBC) implicit solvent MD simulation and the integrated tempering sampling technique. Conformational analysis revealed the different local thermal stabilities for these two peptides, which suggested the different folding landscapes. Our study further suggested that the ellipticities of the isotope peaks in the coherent IR signals are more sensitive to these local stability differences compared with other spectral features such as the peak intensities. Our technique can thus be combined with the relevant experimental measurements to achieve a better understanding of the peptide folding behaviors.

High Thermoelectric Performance of New Rhombohedral Phase of GeSe stabilized through Alloying with AgSbSe2

GeSe is a IV-VI semiconductor, like the excellent thermoelectric materials PbTe and SnSe. Orthorhombic GeSe has been predicted theoretically to have good thermoelectric performance but is difficult to dope experimentally. Like PbTe, rhombohedral GeTe has a multivalley band structure, which is ideal for thermoelectrics and also promotes the formation of Ge vacancies to provide enough carriers for electrical transport. Herein, we investigate the thermoelectric properties of GeSe alloyed with AgSbSe2 , which stabilizes a new rhombohedral structure with higher symmetry that leads to a multivalley Fermi surface and a dramatic increase in carrier concentration. The zT of GeAg0.2 Sb0.2 Se1.4 reaches 0.86 at 710 K, which is 18 times higher than that of pristine GeSe and over four times higher than doped orthorhombic GeSe. Our results open a new avenue towards developing novel thermoelectric materials through crystal phase engineering using a strategy of entropy stabilization of high-symmetry alloys.

Comparison Studies on Sub-Nanometer-Sized Ion Clusters in Aqueous Solutions: Vibrational Energy Transfers, MD Simulations, and Neutron Scattering

In this work, MD simulations with two different force fields, vibrational energy relaxation and resonant energy transfer experiments, and neutron scattering data are used to investigate ion pairing and clustering in a series of GdmSCN aqueous solutions. The MD simulations reproduce the major features of neutron scattering experimental data very well. Although no information about ion pairing or clustering can be obtained from the neutron scattering data, MD calculations clearly demonstrate that substantial amounts of ion pairs and small ion clusters (subnanometers to a few nanometers) do exist in the solutions of concentrations 0.5 M*, 3 M*, and 5 M* (M* denotes mole of GdmSCN per 55.55 mole of water). Vibrational relaxation experiments suggest that significant amounts of ion pairs form in the solutions. Experiments measuring the resonant energy transfers among the thiocyanate anions in the solutions suggest that the ions form clusters and in the clusters the average anion distance is 5.6 Å (5.4 Å) in the 3 M* (5 M*) Gdm-DSCN/D2O solution.

Dramatically enhanced thermoelectric performance of MoS2 by introducing MoO2 nanoinclusions

Two-dimensional transition-metal dichalcogenide semiconductors (TMDCs) with layered structures, such as MoS2, hold great potential to become economic and nontoxic thermoelectric materials. Application of TMDCs is hampered, however, by their insignificant power factors which cancel the advantage of their intrinsically low thermal conductivities along the cross-plane direction and lead to less satisfactory overall thermoelectric performances. Here we report that, by adopting an oxygen doping strategy, the thermoelectric efficiency of MoS2 can be enhanced up to 50 times with the best performance appearing along the cross-plane direction. Our further characterization suggests that this plausible improvement originates from the MoO2 nanoinclusions, which enhance the electrical conductivity and Seebeck coefficient, while suppressing the thermal conductivity at the same time. The unexpected simultaneous enhancement of the electrical conductivity and Seebeck coefficient after doping is explained using an electron relaxation time model. We therefore provide a general strategy towards improving the thermoelectric performance of TMDCs.

Ion effect on the dynamics of water hydrogen bonding network: A theoretical and computational spectroscopy point of view

Understanding the elementary hydration dynamics of the ions and their couplings with the aqueous environments is important for researches in fields including energy, molecular biology, biomedical sciences, and chemical reactions. The concept of hydration shell is ubiquitously used to rationalize the differences of water behavior near the ions with respect to those in the bulk water. One intriguing issue, however, is to decide the spatial range of these hydration shells. Different experimental spectroscopic techniques such as the femtosecond infrared at high frequency and optical Kerr effect, dielectric relaxation, as well as terahertz measurements at low frequency often give controversial indications on this matter. As the optical transition observables provided by the spectroscopic measurements only indirectly reflect the real‐space structure and dynamics information, the theoretical modeling of these signals is often desired in order to reveal the underlying physics in each of these measurements, and to understand the aforementioned apparent controversy. In this review, we outline recent progresses in the theoretical modeling of water dynamics around the ions and ionic moieties and the related vibrational spectroscopy, especially on the femtosecond infrared related single molecular water reorientation and the low‐frequency vibrational spectra related collective water dynamics.

Realizing p-Type MoS2 with Enhanced Thermoelectric Performance by Embedding VMo2S4 Nanoinclusions

Two-dimensional transition-metal dichalcogenide semiconductors (TMDCs) such as MoS2 are attracting increasing interest as thermoelectric materials owing to their abundance, nontoxicity, and promising performance. Recently, we have successfully developed n-type MoS2 thermoelectric material via oxygen doping. Nevertheless, an efficient thermoelectric module requires both n-type and p-type materials with similar compatibility factors. Here, we present a facile approach to obtain a p-type MoS2 thermoelectric material with a maximum figure of merit of 0.18 through the introduction of VMo2S4 as a second phase by vanadium doping. VMo2S4 nanoinclusions, confirmed by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) measurements, not only improve the electrical conductivity by simultaneously increasing the carrier concentration and the mobility but also result in the reduction of lattice thermal conductivity by enhancing the interface phonon scattering. Our studies not only shed new light toward improving thermoelectric performance of TMDCs by a facile elemental doping strategy but also pave the way toward thermoelectric devices based on TMDCs.

Modeling the temperature-dependent peptide vibrational spectra based on implicit-solvent model and enhance sampling technique

We herein review our studies on simulating the thermal unfolding Fourier transform infrared and two-dimensional infrared spectra of peptides. The peptide‐water configuration ensembles, required forspectrum modeling, aregenerated at a series of temperatures using the GB OBC implicit solvent model and the integrated tempering sampling technique. The fluctuating vibrational Hamiltonians of the amide I vibrational band are constructed using the Frenkel exciton model. The signals are calculated using nonlinear exciton propagation. The simulated spectral features such as the intensity and ellipticity are consistent with the experimental observations. Comparing the signals for two beta-hairpin polypeptides with similar structures suggests that this technique is sensitive to peptide folding landscapes.