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The pursuit of structure–property relationships in crystalline metal halide perovskites (MHPs) has yielded an unprecedented combination of advantageous characteristics for wide-ranging optoelectronic applications. While crystalline MHP structures are readily accessible through diffraction-based structure refinements, providing a clear view of associated long-range ordering, the local structures in more recently discovered glassy MHP states remain unexplored. Herein, we utilize a combination of Raman spectroscopy, solid-state nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy, in situ X-ray diffraction (XRD) and pair distribution function (PDF) analysis to investigate the coordination environment in crystalline, glass and melt states of the 2D MHP [(S)-(−)-1-(1-naphthyl)ethylammonium]2PbBr4. While crystalline SNPB shows polarization-dependent Raman spectra, the glassy and melt states exhibit broad features and lack polarization dependence. Solid-state NMR reveals disordering at the organic–inorganic interface of the glass due to significant spatial disruption in the tethering ammonium groups and the corresponding dihedral bond angles connecting the naphthyl and ammonium groups, while still preserving substantial naphthyl group registry and remnants of the layering from the crystalline state (deduced from XRD analysis). Moreover, PDF analysis demonstrates the persistence of corner-sharing PbBr6octahedra in the inorganic framework of the melt/glass phases, but with a loss of structural coherence over length scales exceeding approximately one octahedron due to disorder in the inter- and intraoctahedra bond angles/lengths. These findings deepen our understanding of diverse MHP structural motifs and how structural alterations within the MHP glass affect properties, offering potential for advancing next-generation phase change materials and devices.

Polar and chiral crystal symmetries confer a variety of potentially useful functionalities upon solids by coupling otherwise noninteracting mechanical, electronic, optical, and magnetic degrees of freedom. We describe two phases of the 3D perovskite, CsSnBr3, which emerge below 85 K due to the formation of Sn(II) lone pairs and their interaction with extant octahedral tilts. Phase II (77 K < T< 85 K, space group P21/m) exhibits ferroaxial order driven by a noncollinear pattern of lone pair-driven distortions within the plane normal to the unique octahedral tilt axis, preserving the inversion symmetry observed at higher temperatures. Phase I (T< 77 K, space group P21) additionally exhibits ferroelectric order due to distortions along the unique tilt axis, breaking both inversion and mirror symmetries. This polar and chiral phase exhibits second harmonic generation from the bulk and pronounced electrostriction and negative thermal expansion along the polar axis (Q22≈ 1.1 m4C–2; αb= −7.8 × 10–5K–1) through the onset of polarization. The structures of phases I and II were predicted by recursively following harmonic phonon instabilities to generate a tree of candidate structures and subsequently corroborated by synchrotron X-ray powder diffraction and polarized Raman and 81Br nuclear quadrupole resonance spectroscopies. Preliminary attempts to suppress unintentional hole doping to allow for ferroelectric switching are described. Together, the polar symmetry, small band gap, large spin–orbit splitting of Sn 5p orbitals, and predicted strain sensitivity of the symmetry-breaking distortions suggest bulk samples and epitaxial films of CsSnBr3or its neighboring solid solutions as candidates for bulk Rashba effects.

We have found that the polarization dependence of Raman scattering in organic crystals at finite temperatures can only be described by a fourth-rank tensor formalism. This generalization of the second-rank Raman tensor Rstems from the effect of off-diagonal components in the crystal self-energy on the light scattering mechanism. We thus establish a novel manifestation of phonon–phonon interaction in inelastic light scattering, markedly separate from the better-known phonon lifetime.

Aliovalent substitution is a common strategy to improve the ionic conductivity of solid electrolytes for solid-state batteries. The substitution of SbS43–by WS42–in Na2.9Sb0.9W0.1S4leads to a very high ionic conductivity of 41 mS cm–1at room temperature. While pristine Na3SbS4crystallizes in a tetragonal structure, the substituted Na2.9Sb0.9W0.1S4crystallizes in a cubic phase at room temperature based on its X-ray diffractogram. Here, we show by performing pair distribution function analyses and static single-pulse 121Sb NMR experiments that the short-range order of Na2.9Sb0.9W0.1S4remains tetragonal despite the change in the Bragg diffraction pattern. Temperature-dependent Raman spectroscopy revealed that changed lattice dynamics due to the increased disorder in the Na+substructure leads to dynamic sampling causing the discrepancy in local and average structure. While showing no differences in the local structure, compared to pristine Na3SbS4, quasi-elastic neutron scattering and solid-state 23Na nuclear magnetic resonance measurements revealed drastically improved Na+diffusivity and decreased activation energies for Na2.9Sb0.9W0.1S4. The obtained diffusion coefficients are in very good agreement with theoretical values and long-range transport measured by impedance spectroscopy. This work demonstrates the importance of studying the local structure of ionic conductors to fully understand their transport mechanisms, a prerequisite for the development of faster ionic conductors.

Metal halide perovskites (MHPs) have unique characteristics and hold great potential for next-generation optoelectronic technologies. Recently, the importance of lattice strain in MHPs has been gaining recognition as a significant optimization parameter for device performance. While the effect of strain on the fundamental properties of MHPs has been at the center of interest, its combined effect with an external electric field has been largely overlooked. Here we perform an electric-field-dependent photoluminescence study on heteroepitaxially strained surface-guided CsPbBr3nanowires. We reveal an unexpected strong linear dependence of the photoluminescence intensity on the alternating field amplitude, stemming from an induced internal dipole. Using low-frequency polarized-Raman spectroscopy, we reveal structural modifications in the nanowires under an external field, associated with the observed polarity. These results reflect the important interplay between strain and an external field in MHPs and offer opportunities for the design of MHP-based optoelectronic nanodevices.

Recent investigations of two-dimensional (2D) hybrid organic–inorganic halide perovskites (HHPs) indicate that their optical and electronic properties are dominated by strong coupling to thermal fluctuations. While the optical properties of 2D-HHPs have been extensively studied, a comprehensive understanding of electron–phonon interactions is limited because little is known about their structural dynamics. This is partially because the unit cells of 2D-HHPs contain many atoms. Therefore, the thermal fluctuations are complex and difficult to elucidate in detail. To overcome this challenge, we use polarization-orientation Raman spectroscopy and ab initiocalculations to compare the structural dynamics of the prototypical 2D-HHPs [(BA)2PbI4and (PhE)2PbI4] to their three-dimensional (3D) counterpart, MAPbI3. Comparison to the simpler, 3D MAPbI3crystal shows clear similarities with the structural dynamics of (BA)2PbI4and (PhE)2PbI4across a wide temperature range. The analogy between the 3D and 2D crystals allows us to isolate the effect of the organic cation on the structural dynamics of the inorganic scaffold of the 2D-HHPs. Furthermore, using this approach, we uncover the mechanism of the order–disorder phase transition of (BA)2PbI4(274 K) and show that it involves relaxation of octahedral tilting coupled to anharmonic thermal fluctuations. These anharmonic fluctuations are important because they induce charge carrier localization and affect the optoelectronic performance of these materials.

Two-dimensional hybrid perovskites are currently in the spotlight of condensed matter and nanotechnology research due to their intriguing optoelectronic and vibrational properties with emerging potential for light-harvesting and light-emitting applications. While it is known that these natural quantum wells host tightly bound excitons, the mobilities of these fundamental optical excitations at the heart of the optoelectronic applications are barely explored. Here, we directly monitor the diffusion of excitons through ultrafast emission microscopy from liquid helium to room temperature in hBN-encapsulated two-dimensional hybrid perovskites. We find very fast diffusion with characteristic hallmarks of free exciton propagation for all temperatures above 50 K. In the cryogenic regime, we observe nonlinear, anomalous behavior with an exceptionally rapid expansion of the exciton cloud followed by a very slow and even negative effective diffusion. We discuss our findings in view of efficient exciton–phonon coupling, highlighting two-dimensional hybrids as promising platforms for basic research and optoelectronic applications.

Real-time vibrational microscopy has been recently demonstrated by various techniques, most of them utilizing the well-known schemes of coherent anti-stokes Raman scattering and stimulated Raman scattering. These techniques readily provide valuable chemical information mostly in the higher vibrational frequency regime (>400  cm^−1). Addressing the low vibrational frequency regime (<200  cm^−1) is challenging due to the usage of spectral filters that are required to isolate the signal from the Rayleigh scattered excitation field. In this Letter, we report on rapid, high-resolution, low-frequency (<130  cm^−1) vibrational microscopy using impulsive coherent Raman excitation. By combining impulsive excitation with a fast acousto-optic delay line, we detect the Raman-induced optical Kerr lensing and spectral shift effects with a 25 μs pixel dwell time to produce shot-noise limited, low-frequency hyper-spectral images of various samples.

Coherent anti-Stokes Raman scattering (CARS) has found wide applications in biomedical research. Compared with alternatives, single-beam CARS is especially attractive at low frequencies. Yet, currently existing schemes necessitate a relatively complicated setup to perform high-resolution spectroscopy. Here we show that the spectral sharp edge formed by an ultra-steep long-pass filter is sufficient for performing CARS spectroscopy, simplifying the system significantly. We compare the sensitivity of the presented methodology with available counterparts both theoretically and experimentally. Importantly, we show that this method, to the best of our knowledge, is the simplest and most suitable for vibrational imaging and spectroscopy in the very low-frequency regime (<200  cm^−1).

Controlling polymorphism, namely, the occurrence of multiple crystal forms for a given compound, is still an open technological challenge that needs to be addressed for the reliable manufacturing of crystalline functional materials. Here, we devised a series of 13 organic crystals engineered to embody molecular fragments undergoing specific nanoscale motion anticipated to drive cooperative order–disorder phase transitions. By combining polarized optical microscopy coupled with a heating/cooling stage, differential scanning calorimetry, X-ray diffraction, low-frequency Raman spectroscopy, and calculations (density functional theory and molecular dynamics), we proved the occurrence of cooperative transitions in all the crystalline systems, and we demonstrated how both the molecular structure and lattice dynamics play crucial roles in these peculiar solid-to-solid transformations. These results introduce an efficient strategy to design polymorphic molecular crystalline materials endowed with specific molecular-scale lattice and macroscopic dynamics.

WS2nanotubes present many new technologies under development, including reinforced biocompatible polymers, membranes, photovoltaic-based memories, ferroelectric devices, etc. These technologies depend on the aspect ratio (length/diameter) of the nanotubes, which was limited to 100 or so. A new synthetic technique is presented, resulting in WS2nanotubes a few hundred micrometers long and diameters below 50 nm (aspect ratios of 2000–5000) in high yields. Preliminary investigation into the mechanistic aspects of the two-step synthesis reveals that W5O14nanowhisker intermediates are formed in the first step of the reaction instead of the ubiquitous W18O49nanowhiskers used in the previous syntheses. The electrical and photoluminescence properties of the long nanotubes were studied. WS2nanotube-based paper-like material was prepared via a wet-laying process, which could not be realized with the 10 μm long WS2nanotubes. Ultrafiltration of gold nanoparticles using the nanotube-paper membrane was demonstrated.

We report on the passivation properties of molecularly modified, oxide-free Si(111) surfaces. The reaction of 1-alcohol with the H-passivated Si(111) surface can follow two possible paths, nucleophilic substitution (SN) and radical chain reaction (RCR), depending on adsorption conditions. Moderate heating leads to the SNreaction, whereas with UV irradiation RCR dominates, with SNas a secondary path. We show that the site-sensitive SNreaction leads to better electrical passivation, as indicated by smaller surface band bending and a longer lifetime of minority carriers. However, the surface-insensitive RCR reaction leads to more dense monolayers and, therefore, to much better chemical stability, with lasting protection of the Si surface against oxidation. Thus, our study reveals an inherent dissonance between electrical and chemical passivation. Alkoxy monolayers, formed under UV irradiation, benefit, though, from both chemical and electronic passivation because under these conditions both SNand RCR occur. This is reflected in longer minority carrier lifetimes, lower reverse currents in the dark, and improved photovoltaic performance, over what is obtained if only one of the mechanisms operates. These results show how chemical kinetics and reaction paths impact electronic properties at the device level. It further suggests an approach for effective passivation of other semiconductors.

The interface level alignment of alkyl and alkenyl monolayers, covalently bound to oxide-free Si substrates of various doping levels, is studied using X-ray photoelectron spectroscopy. Using shifts in the C 1s and Si 2p photoelectron peaks as a sensitive probe, we find that charge distribution around the covalent Si–C bond dipole changes according to the initial position of the Fermi level within the Si substrate. This shows that the interface dipole is not fixed but rather changes with the doping level. These results set limits to the applicability of simple models to describe level alignment at interfaces and show that the interface bond and dipole may change according to the electrostatic potential at the interface.

One of the major problems in molecular electronics is how to make electronically conducting contact to the “soft” organic and biomolecules without altering the molecules. As a result, only a small number of metals can be applied, mostly by special deposition methods with severe limitations. Transferring a predefined thin metal leaf onto a molecular layer provides a nondestructive, noninvasive contacting method that is, in principle, applicable to many types of metal and a variety of metal/molecules combinations. Here we report a modification of our earlier lift-off, float-on (LOFO) method, using as a basis its offspring, the polymer-assisted lift-off (PALO) method, where a backing polymer enables simultaneous deposition of multiple contacts as well as reduces wrinkles in the thin metal leaf. The modified PALO (MoPALO) method, reported here, adds lithography steps to obviate the need to punch through the polymer, as is done to complete PALO contacts. Morphological characterization of the electrodes indicates highly uniform, wrinkle-free contacts of negligible roughness. The good electrical performance of the MoPALO contacts was proven with metal/organic-monolayer/semiconductor (MOMS) junctions, which are known to be very sensitive to molecular degradation and metal penetration. We also show how MoPALO contacts enabled us to compare the effect of varying the metal work function and contact area on the current−voltage characteristics of MOMS devices.

Using a semiconductor as the substrate to a molecular organic layer, penetration of metal contacts can be clearly identified by the study of electronic charge transport through the layer. A series of monolayers of saturated hydrocarbon molecules with varying lengths is assembled on Si or GaAs and the junctions resulting after further electronic contact is made by liquid Hg, indirect metal evaporation, and a “ready‐made” metal pad are measured. In contrast to tunneling characteristics, which are ambiguous regarding contact penetration, the semiconductor surface barrier is very sensitive to any direct contact with a metal. With the organic monolayer intact, a metal–insulator–semiconductor (MIS) structure results. If metal penetrated the monolayer, the junction behaves as a metal–semiconductor (MS) structure. By comparing a molecule‐free interface (MS junction) with a molecularly modified one (presumably MIS), possible metal penetration is identified. The major indicators are the semiconductor electronic transport barrier height, extracted from the junction transport characteristics, and the photovoltage. The approach does not require a series of different monolayers and data analysis is quite straightforward, helping to identify non‐invasive ways to make electronic contact to soft matter.

Metal−organic molecule−semiconductor junctions are controlled not only by the molecular properties, as in metal−organic molecule−metal junctions, but also by effects of the molecular dipole, the dipolar molecule−semiconductor link, and molecule−semiconductor charge transfer, and by the effects of all these on the semiconductor depletion layer (i.e., on the internal semiconductor barrier to charge transport). Here, we report on and compare the electrical properties (current−voltage, capacitance−voltage, and work function) of large area Hg/organic monolayer-Si junctions with alkyl and alkenyl monolayers on moderately and highly doped n-Si, and combine the experimental data with simulations of charge transport and electronic structure calculations. We show that, for moderately doped Si, the internal semiconductor barrier completely controls transport and the attached molecules influence the transport of such junctions only in that they drive the Si into inversion. The resulting minority carrier-controlled junction is not sensitive to molecular changes in the organic monolayer at reverse and low forward bias and is controlled by series resistance at higher forward bias. However, in the case of highly doped Si, the internal barrier is smaller, and as a result, the charge transport properties of the junction are affected by changing from an alkyl to an alkenyl monolayer. We propose that the double bond near the surface primarily increases the coupling between the organic monolayer and the Si, which increases the current density at a given bias by increasing the contact conductance.