Hybrid Germanium Iodide Perovskite Semiconductors: Active Lone Pairs, Structural Distortions, Direct and Indirect Energy Gaps, and Strong Nonlinear Optical Properties

The synthesis and properties of the hybrid organic/inorganic germanium perovskite compounds, AGeI₃, are reported (A = Cs, organic cation). The systematic study of this reaction system led to the isolation of 6 new hybrid semiconductors. Using CsGeI₃ (<b>1</b>) as the prototype compound, we have prepared methylammonium, CH₃NH₃GeI₃ (<b>2</b>), formamidinium, HC­(NH₂)₂GeI₃ (<b>3</b>), acetamidinium, CH₃C­(NH₂)₂GeI₃ (<b>4</b>), guanidinium, C­(NH₂)₃GeI₃ (<b>5</b>), trimethylammonium, (CH₃)₃NHGeI₃ (<b>6</b>), and isopropylammonium, (CH₃)₂C­(H)­NH₃GeI₃ (<b>7</b>) analogues. The crystal structures of the compounds are classified based on their dimensionality with <b>1</b>–<b>4</b> forming 3D perovskite frameworks and <b>5</b>–<b>7</b> 1D infinite chains. Compounds <b>1</b>–<b>7</b>, with the exception of compounds <b>5</b> (centrosymmetric) and <b>7</b> (nonpolar acentric), crystallize in polar space groups. The 3D compounds have direct band gaps of 1.6 eV (<b>1</b>), 1.9 eV (<b>2</b>), 2.2 eV (<b>3</b>), and 2.5 eV (<b>4</b>), while the 1D compounds have indirect band gaps of 2.7 eV (<b>5</b>), 2.5 eV (<b>6</b>), and 2.8 eV (<b>7</b>). Herein, we report on the second harmonic generation (SHG) properties of the compounds, which display remarkably strong, type I phase-matchable SHG response with high laser-induced damage thresholds (up to ∼3 GW/cm²). The second-order nonlinear susceptibility, χ_S⁽²⁾, was determined to be 125.3 ± 10.5 pm/V (<b>1</b>), (161.0 ± 14.5) pm/V (<b>2</b>), 143.0 ± 13.5 pm/V (<b>3</b>), and 57.2 ± 5.5 pm/V (<b>4</b>). First-principles density functional theory electronic structure calculations indicate that the large SHG response is attributed to the high density of states in the valence band due to sp-hybridization of the Ge and I orbitals, a consequence of the lone pair activation

Hybrid Germanium Iodide Perovskite Semiconductors: Active Lone Pairs, Structural Distortions, Direct and Indirect Energy Gaps, and Strong Nonlinear Optical Properties

The synthesis and properties of the hybrid organic/inorganic germanium perovskite compounds, AGeI₃, are reported (A = Cs, organic cation). The systematic study of this reaction system led to the isolation of 6 new hybrid semiconductors. Using CsGeI₃ (<b>1</b>) as the prototype compound, we have prepared methylammonium, CH₃NH₃GeI₃ (<b>2</b>), formamidinium, HC­(NH₂)₂GeI₃ (<b>3</b>), acetamidinium, CH₃C­(NH₂)₂GeI₃ (<b>4</b>), guanidinium, C­(NH₂)₃GeI₃ (<b>5</b>), trimethylammonium, (CH₃)₃NHGeI₃ (<b>6</b>), and isopropylammonium, (CH₃)₂C­(H)­NH₃GeI₃ (<b>7</b>) analogues. The crystal structures of the compounds are classified based on their dimensionality with <b>1</b>–<b>4</b> forming 3D perovskite frameworks and <b>5</b>–<b>7</b> 1D infinite chains. Compounds <b>1</b>–<b>7</b>, with the exception of compounds <b>5</b> (centrosymmetric) and <b>7</b> (nonpolar acentric), crystallize in polar space groups. The 3D compounds have direct band gaps of 1.6 eV (<b>1</b>), 1.9 eV (<b>2</b>), 2.2 eV (<b>3</b>), and 2.5 eV (<b>4</b>), while the 1D compounds have indirect band gaps of 2.7 eV (<b>5</b>), 2.5 eV (<b>6</b>), and 2.8 eV (<b>7</b>). Herein, we report on the second harmonic generation (SHG) properties of the compounds, which display remarkably strong, type I phase-matchable SHG response with high laser-induced damage thresholds (up to ∼3 GW/cm²). The second-order nonlinear susceptibility, χ_S⁽²⁾, was determined to be 125.3 ± 10.5 pm/V (<b>1</b>), (161.0 ± 14.5) pm/V (<b>2</b>), 143.0 ± 13.5 pm/V (<b>3</b>), and 57.2 ± 5.5 pm/V (<b>4</b>). First-principles density functional theory electronic structure calculations indicate that the large SHG response is attributed to the high density of states in the valence band due to sp-hybridization of the Ge and I orbitals, a consequence of the lone pair activation

Coherent Lattice Vibrations in Mono- and Few-Layer WSe₂

We report the observation of coherent lattice vibrations in mono- and few-layer WSe₂ in the time domain, which were obtained by performing time-resolved transmission measurements. Upon the excitation of ultrashort pulses with the energy resonant to that of <i>A</i> excitons, coherent oscillations of the A_1g optical phonon and longitudinal acoustic phonon at the M point of the Brillouin zone (LA­(M)) were impulsively generated in monolayer WSe₂. In multilayer WSe₂ flakes, the interlayer breathing mode (B₁) is found to be sensitive to the number of layers, demonstrating its usefulness in characterizing layered transition metal dichalcogenide materials. On the basis of temperature-dependent measurements, we find that the A_1g optical phonon mode decays into two acoustic phonons through the anharmonic decay process

Impact of Selenium Doping on Resonant Second-Harmonic Generation in Monolayer MoS₂

We have investigated strong optical nonlinearity of monolayer MoS_{2(1–<i>x</i>)}Se_2<i>x</i> across the exciton resonance, which is directly tunable by Se doping. The quality of monolayer alloys prepared by chemical vapor deposition is verified by atomic force microscopy, Raman spectroscopy, and photoluminescence analysis. The crystal symmetry of all of our alloys is essentially <i>D</i>_3<i>h</i>, as confirmed by polarization-dependent second-harmonic generation (SHG). The spectral structure of the exciton resonance is sampled by wavelength-dependent SHG (λ = 1000–1800 nm), where the SHG resonance red-shifts in accordance with the corresponding optical gap. Surprisingly, the effect of compositional variation turns out to be much more dramatic owing to the unexpected increase of <i>B</i>-exciton-induced SHG, which indeed dominates over the <i>A</i>-exciton resonance for <i>x</i> ≥ 0.3. The overall effect is therefore stronger and broader SHG resonance where the latter arises from different degrees of red-shift for the two exciton states. We report the corresponding absolute SHG dispersion of monolayer alloys, χ⁽²⁾, as a function of Se doping. We believe that our finding is a critical step toward engineering highly efficient nonlinear optical van der Waals materials working in a broader performance range

Growth and Simultaneous Valleys Manipulation of Two-Dimensional MoSe₂‑WSe₂ Lateral Heterostructure

The covalently bonded in-plane heterostructure (HS) of monolayer transition-metal dichalcogenides (TMDCs) possesses huge potential for high-speed electronic devices in terms of valleytronics. In this study, high-quality monolayer MoSe₂-WSe₂ lateral HSs are grown by pulsed-laser-deposition-assisted selenization method. The sharp interface of the lateral HS is verified by morphological and optical characterizations. Intriguingly, photoluminescence spectra acquired from the interface show rather clear signatures of pristine MoSe₂ and WSe₂ with no intermediate energy peak related to intralayer excitonic matter or formation of Mo_<i>x</i>W_{(1–<i>x</i>)}Se₂ alloys, thereby confirming the sharp interface. Furthermore, the discrete nature of laterally attached TMDC monolayers, each with doubly degenerated but nonequivalent energy valleys marked by (<i>K</i>_M, <i>K</i>′_M) for MoSe₂ and (<i>K</i>_W, <i>K</i>′_W) for WSe₂ in <i>k</i> space, allows simultaneous control of the four valleys within the excitation area without any crosstalk effect over the interface. As an example, <i>K</i>_M and <i>K</i>_W valleys or <i>K</i>′_M and <i>K</i>′_W valleys are simultaneously polarized by controlling the helicity of circularly polarized optical pumping, where the maximum degree of polarization is achieved at their respective band edges. The current work provides the growth mechanism of laterally sharp HSs and highlights their potential use in valleytronics