What Limits the Intrinsic Mobility of Electrons and Holes in Two Dimensional Metal Dichalcogenides?

Nonlinear optical response spatial self-phase modulation in MoTe2: correlations between χ(3) and mobility or effective mass

We report on an unambiguous observation of the third-order nonlinear optical effect, spatial self-phase modulation (SSPM), in a MoTe₂ dispersion. The values of the third-order nonlinear optical coefficients effectively for one-layer MoTe₂, χone-layer(3), are obtained through the SSPM method at excitation wavelengths 473, 532, 750, and 801 nm, respectively. The wind-chime model is used to explain the ring formation time. The wavelength dependence of χone-layer(3) compares well with the photo-absorption spectra. Significantly, we find a correlation between χ⁽³⁾ and the carrier mobility μ or effective mass m*, which again further supports the laser-induced ac electron coherence in 2D materials.

Comprehensive understanding of intrinsic mobility in the monolayers of III-VI group 2D materials

Monolayers of III-VI group two-dimensional (2D) materials MX (M = Ga and In and X = S, Se, and Te) have attracted global interest for potential applications in electronic and photoelectric devices due to their attractive physical and chemical characteristics. However, a comprehensive understanding of the distinguished carrier mobility in MX monolayers is of great importance and not yet clear. Herein, using a Boltzmann transport equation (BTE) solver and first principles calculations, we have precisely revealed that the intrinsic mobility in MX monolayers is significantly limited by phonon scattering. Note that the longitudinal acoustic phonon mode and optic phonon modes and were found predominantly coupled with electrons, which strongly restrained the intrinsic mobility in the MX monolayers. Interestingly, apart from a moderate band gap, the GaSe and GaTe monolayers exhibit high electron mobility exceeding 103 cm2 V-1 s-1 and may serve as outstanding electron transport channels. We believe that our findings will shed light on the design and applications of MX monolayers and 2D materials in nanoscale electronic and photoelectric devices.

The Optimal Electronic Structure for High-Mobility 2D Semiconductors: Exceptionally High Hole Mobility in 2D Antimony

Two-dimensional (2D) semiconductors have very attractive properties for many applications such as photoelectrochemistry. However, a significant challenge that limits their further developments is the relatively low electron/hole mobility at room temperature. Here using the Boltzmann transport theory with the scattering rates calculated from first-principles that allow us to accurately determine the mobility, we discover an exceptionally high intrinsic mobility of holes in monolayer antimony (Sb), which is ∼1330 cm² V^-1 s^-1 at room temperature, much higher than the common 2D semiconductors including MoS₂, InSe, and black phosphorus in monolayer form, and is the highest among 2D materials with a band gap of >1 eV reported so far. Its high mobility and the moderate band gap make it very promising for many applications. By comparing the 2D Sb with other 2D materials in the same group, we find that the high mobility is closely related with its electronic structure, which has a sharp and deep valence band valley, and, importantly, located at the Γ point. This electronic structure not only gives rise to a high velocity for charge carriers but also leads to a small density of states for accepting the scattered carriers, particularly by eliminating the valley-valley and peak-valley scatterings that are found to be significant for other materials. This type of electronic structure thus can be used as a target feature to design/discover high-mobility 2D semiconductors. Our work provides a promising material to overcome the mobility issue and also suggests a simple and general principle for high-mobility semiconductor design/discovery.

The origin of intrinsic charge transport for Dirac carbon sheet materials: roles of acetylenic linkage and electron-phonon couplings

Within the past few years, intriguing graphene Dirac cones have attracted intense interest in novel two-dimensional (2D) Dirac materials as ultrahigh-mobility functional materials. In this work, the phonon-limited charge transport properties of α-graphyne (α-GY), α-graphdiyne (α-GDY), and β-graphyne (β-GY) were investigated using the Boltzmann transport equation within the first-principles framework while considering the electron-phonon coupling (EPC). Despite all three investigated compounds being 2D Dirac carbon materials, each demonstrated distinctly different carrier mobilities by one order of magnitude (2.2 × 104 cm2 V-1 s-1 for α-GY, 2.1 × 103 cm2 V-1 s-1 for α-GDY and 1.9 × 103 cm2 V-1 s-1 for β-GY at room-temperature and a carrier connection of n ∼ 3 × 1012 cm-2). The essential differences in the mobilities of these materials originated from the acetylenic linkage limiting the group velocity and the E2g phonon modes limiting the scattering time. For example, a few uniformly equivalent acetylenic linkages and E2g phonon modes tend to generate high mobilities. A simple mobility relationship was determined using the number of E2g photon modes, allowing for a quick estimation of the mobilities for Dirac materials. α-GY was identified as a promising alternative to graphene for next generation nanoelectronic devices.

Charge Mobility and Recombination Mechanisms in Tellurium van der Waals Solid

Trigonal tellurium is a small band gap elemental semiconductor consisting of van der Waals bound one-dimensional helical chains of tellurium atoms. We study the temperature dependence of the charge carrier mobility and recombination pathways in bulk tellurium. Electrons and holes are generated by irradiation of the sample with 3 MeV electrons and detected by time-resolved microwave conductivity measurements. A theoretical model is used to explain the experimental observations for different charge densities and temperatures. Our analysis reveals a high room temperature mobility of 190 ± 20 cm² V^-1 s^-1. The mobility is thermally deactivated, suggesting a band-like transport mechanism. According to our analysis, the charges predominantly recombine via radiative recombination with a radiative yield close to 98%, even at room temperature. The remaining charges recombine by either trap-assisted (Shockley-Read-Hall) recombination or undergo trapping to deep traps. The high mobility, near-unity radiative yield, and possibility of large-scale production of atomic wires by liquid exfoliation make Te of high potential for next-generation nanoelectronic and optoelectronic applications, including far-infrared detectors and lasers.

Sudden polarization and zwitterion formation as a pseudo-Jahn–Teller effect: a new insight into the photochemistry of alkenes

We show that the intermediates of photochemical reactions—sudden polarization and zwitterion formations—are consequences of the pseudo-Jahn–Teller effect (PJTE), which facilitates a better understanding, rationalization, prediction, and manipulation of the corresponding chemical and biological processes.