Anisotropic Optical and Vibrational Properties of GeS

Tailoring Ultrafast Near-Band Gap Photoconductive Response in GeS by Zero-Valent Cu Intercalation

Zero-valent intercalation of atomic metals into the van der Waals gap of layered materials can be used to tune their electronic, optical, thermal, and mechanical properties. Here, we report the impact of intercalating ∼3 atm percent of zero-valent copper into germanium sulfide (GeS). Advanced many-body calculations predict that copper introduces quasi-localized intermediate band states, and time-resolved THz spectroscopy studies demonstrate that those states have prominent effects on the photoconductivity of GeS. Cu-intercalated GeS exhibits a faster rise of transient photoconductivity and a shorter lifetime of optically injected carriers following near-gap excitation with 800 nm pulses. At the same time, Cu intercalation improves free carrier mobility from 1100 to 1300 cm2 V-1 s-1, which we attribute to the damping of acoustic phonons observed in Brillouin scattering and consequent reduction of phonon scattering.

Optical properties of orthorhombic germanium sulfide: unveiling the anisotropic nature of Wannier excitons

To fully explore exciton-based applications and improve their performance, it is essential to understand the exciton behavior in anisotropic materials. Here, we investigate the optical properties of anisotropic excitons in GeS encapsulated by h-BN using different approaches that combine polarization- and temperature-dependent photoluminescence (PL) measurements, ab initio calculations, and effective mass approximation (EMA). Using the Bethe-Salpeter Equation (BSE) method, we found that the optical absorption spectra in GeS are significantly affected by the Coulomb interaction included in the BSE method, which shows the importance of excitonic effects besides it exhibits a significant dependence on the direction of polarization, revealing the anisotropic nature of bulk GeS. By combining ab initio calculations and EMA methods, we investigated the quasi-hydrogenic exciton states and oscillator strength (OS) of GeS along the zigzag and armchair axes. We found that the anisotropy induces lifting of the degeneracy and mixing of the excitonic states in GeS, which results in highly non-hydrogenic features. A very good agreement with the experiment is observed.

The effect of temperature and excitation energy on Raman scattering in bulkHfS2

Raman scattering (RS) in bulk hafnium disulfide (HfS2) is investigated as a function of temperature (5 K - 350 K) with polarization resolution and excitation of several laser energies. An unexpected temperature dependence of the energies of the main Raman-active (A1gand E_g) modes with the temperature-induced blueshift in the low-temperature limit is observed. The low-temperature quenching of a modeω₁(134 cm^-1) and the emergence of a new mode at approx. 184 cm^-1, labeledZ, is reported. The optical anisotropy of the RS inHfS2is also reported, which is highly susceptible to the excitation energy. The apparent quenching of the A1gmode atT = 5 K and of the E_gmode atT= 300 K in the RS spectrum excited with 3.06 eV excitation is also observed. We discuss the results in the context of possible resonant character of light-phonon interactions. Analyzed is also a possible effect of the iodine molecules intercalated in the van der Waals gaps between neighboringHfS2layers, which inevitably result from the growth procedure.

Resonant Exciton Scattering Reveals Raman Forbidden Phonon Modes in Layered GeS

Germanium monosulfide with an anisotropic puckered crystalline structure has recently attracted much attention due to its unique optical and electronic properties; however, exciton-phonon interactions were only superficially elucidated. We study the resonant Raman scattering and the photoluminescence of the optically active Γ-exciton in layered GeS flakes and evaluate the exciton and phonon responses on variations in the excitation energy, laser-light and emission polarizations, temperature, and laser power. A double-resonance mechanism allows for observing Raman forbidden (dark) first- and second-order longitudinal-optical phonon modes whose symmetries and energies are moreover calculated by density functional perturbation theory. For (quasi)-resonant exciton excitation, the selection rules become relaxed so that a fourth-order Fröhlich intraband process is mediated by the scattering of the electron with a longitudinal-optical and an acoustic phonon. Our results demonstrate considerable coupling between phonons and photogenerated carriers in GeS flakes and the high efficiency of multiorder scattering in optical processes.