Disentangling the effects of structure and lone-pair electrons in the lattice dynamics of halide perovskites

Halide perovskites show great optoelectronic performance, but their favorable properties are paired with unusually strong anharmonicity. It was proposed that this combination derives from the ns2 electron configuration of octahedral cations and associated pseudo-Jahn-Teller effect. We show that such cations are not a prerequisite for the strong anharmonicity and low-energy lattice dynamics encountered in these materials. We combine X-ray diffraction, infrared and Raman spectroscopies, and molecular dynamics to contrast the lattice dynamics of CsSrBr3 with those of CsPbBr3, two compounds that are structurally similar but with the former lacking ns2 cations with the propensity to form electron lone pairs. We exploit low-frequency diffusive Raman scattering, nominally symmetry-forbidden in the cubic phase, as a fingerprint of anharmonicity and reveal that low-frequency tilting occurs irrespective of octahedral cation electron configuration. This highlights the role of structure in perovskite lattice dynamics, providing design rules for the emerging class of soft perovskite semiconductors.

Submillimeter-Long WS2 Nanotubes: The Pathway to Inorganic Buckypaper

WS2 nanotubes 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 WS2 nanotubes 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 W5O14 nanowhisker intermediates are formed in the first step of the reaction instead of the ubiquitous W18O49 nanowhiskers used in the previous syntheses. The electrical and photoluminescence properties of the long nanotubes were studied. WS2 nanotube-based paper-like material was prepared via a wet-laying process, which could not be realized with the 10 μm long WS2 nanotubes. Ultrafiltration of gold nanoparticles using the nanotube-paper membrane was demonstrated.

Phonon-Phonon Interactions in the Polarization Dependence of Raman Scattering

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 stems 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.

Mobile Trions in Electrically Tunable 2D Hybrid Perovskites

2D hybrid perovskites are currently in the spotlight of material research for light-harvesting and -emitting applications. It remains extremely challenging, however, to externally control their optical response due to the difficulties of introducing electrical doping. Here, an approach of interfacing ultrathin sheets of perovskites with few-layer graphene and hexagonal boron nitride into gate-tunable, hybrid heterostructures, is demonstrated. It allows for bipolar, continuous tuning of light emission and absorption in 2D perovskites by electrically injecting carriers to densities as high as 10¹²  cm^-2 . This reveals the emergence of both negatively and positively charged excitons, or trions, with binding energies up to 46 meV, among the highest measured for 2D systems. Trions are shown to dominate light emission and propagate with mobilities reaching 200 cm² V^-1 s^-1 at elevated temperatures. The findings introduce the physics of interacting mixtures of optical and electrical excitations to the broad family of 2D inorganic-organic nanostructures. The presented strategy to electrically control the optical response of 2D perovskites highlights it as a promising material platform toward electrically modulated light-emitters, externally guided charged exciton currents, and exciton transistors based on layered, hybrid semiconductors.

On the Discrepancy between Local and Average Structure in the Fast Na+ Ionic Conductor Na2.9Sb0.9W0.1S4

Aliovalent substitution is a common strategy to improve the ionic conductivity of solid electrolytes for solid-state batteries. The substitution of SbS₄^3- by WS₄^2- in Na_2.9Sb_0.9W_0.1S₄ leads to a very high ionic conductivity of 41 mS cm^-1 at room temperature. While pristine Na₃SbS₄ crystallizes in a tetragonal structure, the substituted Na_2.9Sb_0.9W_0.1S₄ crystallizes 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 Na_2.9Sb_0.9W_0.1S₄ remains 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 Na_2.9Sb_0.9W_0.1S₄. 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.

Mechanistic View on the Order-Disorder Phase Transition in Amphidynamic Crystals

We combine temperature-dependent low-frequency Raman measurements and first-principles calculations to obtain a mechanistic understanding of the order-disorder phase transition of 2,7-di-tert-butylbenzo[b]benzo[4,5]thieno[2,3-d]thiophene (ditBu-BTBT) and 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) semiconducting amphidynamic crystals. We identify the lattice normal modes associated with the phase transition by following the position and width of the Raman peaks with temperature and identifying peaks that exhibit nonlinear dependence toward the phase transition temperature. Our findings are interpreted according to the "hardcore mode" model previously used to describe order-disorder phase transitions in inorganic and hybrid crystals with a Brownian sublattice. Within the framework of this model, ditBu-BTBT exhibits an ideal behavior where only one lattice mode is associated with the phase transition. TIPS-pentacene deviates strongly from the model due to strong interactions between lattice modes. We discuss the origin of the different behaviors and suggest side-chain engineering as a tool to control polymorphism in amphidynamic crystals.

Static and Dynamic Disorder in Formamidinium Lead Bromide Single Crystals

We show that formamidinium-based crystals are distinct from methylammonium-based halide perovskite crystals because their inorganic sublattice exhibits intrinsic local static disorder that coexists with a well-defined average crystal structure. Our study combines terahertz-range Raman scattering with single-crystal X-ray diffraction and first-principles calculations to probe the evolution of inorganic sublattice dynamics with temperature in the range of 10-300 K. The temperature evolution of the Raman spectra shows that low-temperature, local static disorder strongly affects the crystal structural dynamics and phase transitions at higher temperatures.

Tapered Optical Fibers Coated with Rare-Earth Complexes for Quantum Applications

Crystals and fibers doped with rare-earth (RE) ions provide the basis for most of today's solid-state optical systems, from lasers and telecom devices to emerging potential quantum applications such as quantum memories and optical to microwave conversion. The two platforms, doped crystals and doped fibers, seem mutually exclusive, each having its own strengths and limitations, the former providing high homogeneity and coherence and the latter offering the advantages of robust optical waveguides. Here we present a hybrid platform that does not rely on doping but rather on coating the waveguide-a tapered silica optical fiber-with a monolayer of complexes, each containing a single RE ion. The complexes offer an identical, tailored environment to each ion, thus minimizing inhomogeneity and allowing tuning of their properties to the desired application. Specifically, we use highly luminescent Yb3+[Zn(II)MC (QXA)] complexes, which isolate the RE ion from the environment and suppress nonradiative decay channels. We demonstrate that the beneficial optical transitions of the Yb3+ are retained after deposition on the tapered fiber and observe an excited-state lifetime of over 0.9 ms, on par with state-of-the-art Yb-doped inorganic crystals.

Chemical Modifications Suppress Anharmonic Effects in the Lattice Dynamics of Organic Semiconductors

The lattice dynamics of organic semiconductors has a significant role in determining their electronic and mechanical properties. A common technique to control these macroscopic properties is to chemically modify the molecular structure. These modifications are known to change the molecular packing, but their effect on the lattice dynamics is relatively unexplored. Therefore, we investigate how chemical modifications to a core [1]benzothieno[3,2-b]benzothiophene (BTBT) semiconducting crystal affect the evolution of the crystal structural dynamics with temperature. Our study combines temperature-dependent polarization-orientation (PO) low-frequency Raman measurements with first-principles calculations and single-crystal X-ray diffraction measurements. We show that chemical modifications can indeed suppress specific expressions of vibrational anharmonicity in the lattice dynamics. Specifically, we detect in BTBT a gradual change in the PO Raman response with temperature, indicating a unique anharmonic expression. This anharmonic expression is suppressed in all examined chemically modified crystals (ditBu-BTBT and diC8-BTBT, diPh-BTBT, and DNTT). In addition, we observe solid–solid phase transitions in the alkyl-modified BTBTs. Our findings indicate that π-conjugated chemical modifications are the most effective in suppressing these anharmonic effects.

Anharmonic Lattice Dynamics in Sodium Ion Conductors

We employ terahertz-range temperature-dependent Raman spectroscopy and first-principles lattice dynamical calculations to show that the undoped sodium ion conductors Na₃PS₄ and isostructural Na₃PSe₄ both exhibit anharmonic lattice dynamics. The anharmonic effects in the compounds involve coupled host lattice-Na⁺ ion dynamics that drive the tetragonal-to-cubic phase transition in both cases, but with a qualitative difference in the anharmonic character of the transition. Na₃PSe₄ shows an almost purely displacive character with the soft modes disappearing in the cubic phase as the change in symmetry shifts these modes to the Raman-inactive Brillouin zone boundary. Na₃PS₄ instead shows an order-disorder character in the cubic phase, with the soft modes persisting through the phase transition and remaining Raman active in the cubic phase, violating Raman selection rules for that phase. Our findings highlight the important role of coupled host lattice-mobile ion dynamics in vibrational instabilities that are coincident with the exceptional conductivity of these Na⁺ ion conductors.

Diverging Expressions of Anharmonicity in Halide Perovskites

Lead-based halide perovskite crystals are shown to have strongly anharmonic structural dynamics. This behavior is important because it may be the origin of their exceptional photovoltaic properties. The double perovskite, Cs₂ AgBiBr₆ , has been recently studied as a lead-free alternative for optoelectronic applications. However, it does not exhibit the excellent photovoltaic activity of the lead-based halide perovskites. Therefore, to explore the correlation between the anharmonic structural dynamics and optoelectronic properties in lead-based halide perovskites, the structural dynamics of Cs₂ AgBiBr₆ are investigated and are compared to its lead-based analog, CsPbBr₃ . Using temperature-dependent Raman measurements, it is found that both materials are indeed strongly anharmonic. Nonetheless, the expression of their anharmonic behavior is markedly different. Cs₂ AgBiBr₆ has well-defined normal modes throughout the measured temperature range, while CsPbBr₃ exhibits a complete breakdown of the normal-mode picture above 200 K. It is suggested that the breakdown of the normal-mode picture implies that the average crystal structure may not be a proper starting point to understand the electronic properties of the crystal. In addition to our main findings, an unreported phase of Cs₂ AgBiBr₆ is also discovered below ≈37 K.

Remanent Polarization and Strong Photoluminescence Modulation by an External Electric Field in Epitaxial CsPbBr3 Nanowires

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 CsPbBr₃ nanowires. 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.

Strongly Anharmonic Octahedral Tilting in Two-Dimensional Hybrid Halide Perovskites

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 initio calculations to compare the structural dynamics of the prototypical 2D-HHPs [(BA)2PbI4 and (PhE)2PbI4] to their three-dimensional (3D) counterpart, MAPbI3. Comparison to the simpler, 3D MAPbI3 crystal shows clear similarities with the structural dynamics of (BA)2PbI4 and (PhE)2PbI4 across 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.

Fast and Anomalous Exciton Diffusion in Two-Dimensional Hybrid Perovskites

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.

Revealing Excitonic Phonon Coupling in (PEA)2(MA)n-1PbnI3n+1 2D Layered Perovskites

The family of 2D Ruddlesden-Popper perovskites is currently attracting great interest of the scientific community as highly promising materials for energy harvesting and light emission applications. Despite the fact that these materials are known for decades, only recently has it become apparent that their optical properties are driven by the exciton-phonon coupling, which is controlled by the organic spacers. However, the detailed mechanism of this coupling, which gives rise to complex absorption and emission spectra, is the subject of ongoing controversy. In this work we show that the particularly rich, absorption spectra of (PEA)₂(CH₃NH₃)_n-1Pb_nI_3n+1 (where PEA stands for phenylethylammonium and n = 1, 2, 3), are related to a vibronic progression of excitonic transition. In contrast to other two-dimensional perovskites, we observe a coupling to a high-energy (40 meV) phonon mode probably related to the torsional motion of the NH₃⁺ head of the organic spacer.

Anharmonic Lattice Vibrations in Small-Molecule Organic Semiconductors

The intermolecular lattice vibrations in small-molecule organic semiconductors have a strong impact on their functional properties. Existing models treat the lattice vibrations within the harmonic approximation. In this work, polarization-orientation (PO) Raman measurements are used to monitor the temperature-evolution of the symmetry of lattice vibrations in anthracene and pentacene single crystals. Combined with first-principles calculations, it is shown that at 10 K, the lattice dynamics of the crystals are indeed harmonic. However, as the temperature is increased, specific lattice modes gradually lose their PO dependence and become more liquid-like. This finding is indicative of a dynamic symmetry breaking of the crystal structure and shows clear evidence of the strongly anharmonic nature of these vibrations. Pentacene also shows an apparent phase transition between 80 and 150 K, indicated by a change in the vibrational symmetry of one of the lattice modes. These findings lay the groundwork for accurate predictions of the electronic properties of high-mobility organic semiconductors at room temperature.

Sub-second hyper-spectral low-frequency vibrational imaging via impulsive Raman excitation

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.

Simplified approach to low-frequency coherent anti-Stokes Raman spectroscopy using a sharp spectral edge filter

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).

Dynamic emission Stokes shift and liquid-like dielectric solvation of band edge carriers in lead-halide perovskites

Lead-halide perovskites have emerged as promising materials for photovoltaic and optoelectronic applications. Their significantly anharmonic lattice motion, in contrast to conventional harmonic semiconductors, presents a conceptual challenge in understanding the genesis of their exceptional optoelectronic properties. Here we report a strongly temperature dependent luminescence Stokes shift in the electronic spectra of both hybrid and inorganic lead-bromide perovskite single crystals. This behavior stands in stark contrast to that exhibited by more conventional crystalline semiconductors. We correlate the electronic spectra with the anti-Stokes and Stokes Raman vibrational spectra. Dielectric solvation theories, originally developed for excited molecules dissolved in polar liquids, reproduce our experimental observations. Our approach, which invokes a classical Debye-like relaxation process, captures the dielectric response originating from the incipient anharmonicity of the LO phonon at about 20 meV (160 cm⁻¹) in the lead-bromide framework. We reconcile this liquid-like model incorporating thermally-activated dielectric solvation with more standard solid-state theories of the emission Stokes shift in crystalline semiconductors.

Lead halide perovskites have unique electronic properties that depend on the crystal’s anharmonicity. Dielectric solvation theories, developed for molecules dissolved in polar liquids, are shown here to reproduce the temperature behavior of carrier solvation in the electronic spectra, implying strongly anharmonic lattice dynamics.

What Remains Unexplained about the Properties of Halide Perovskites?

The notion that halide perovskite crystals (ABX₃ , where X is a halide) exhibit unique structural and optoelectronic behavior deserves serious scrutiny. After decades of steady and half a decade of intense research, the question which attributes of these materials are unusual, is discussed, with an emphasis on the identification of the most important remaining issues. The goal is to stimulate discussion rather than to merely present a community consensus.

Local Polar Fluctuations in Lead Halide Perovskite Crystals

Hybrid lead-halide perovskites have emerged as an excellent class of photovoltaic materials. Recent reports suggest that the organic molecular cation is responsible for local polar fluctuations that inhibit carrier recombination. We combine low-frequency Raman scattering with first-principles molecular dynamics (MD) to study the fundamental nature of these local polar fluctuations. Our observations of a strong central peak in the cubic phase of both hybrid (CH_{3}NH_{3}PbBr_{3}) and all-inorganic (CsPbBr_{3}) lead-halide perovskites show that anharmonic, local polar fluctuations are intrinsic to the general lead-halide perovskite structure, and not unique to the dipolar organic cation. MD simulations indicate that head-to-head Cs motion coupled to Br face expansion, occurring on a few hundred femtosecond time scale, drives the local polar fluctuations in CsPbBr_{3}.

Infrared Spectroscopic Study of Vibrational Modes in Methylammonium Lead Halide Perovskites

The organic cation and its interplay with the inorganic lattice underlie the exceptional optoelectronic properties of organo-metallic halide perovskites. Herein we report high-quality infrared spectroscopic measurements of methylammonium lead halide perovskite (CH3NH3Pb(I/Br/Cl)3) films and single crystals at room temperature, from which the dielectric function in the investigated spectral range is derived. Comparison with electronic structure calculations in vacuum of the free methylammonium cation allows for a detailed peak assignment. We analyze the shifts of the vibrational peak positions between the different halides and infer the extent of interaction between organic moiety and the surrounding inorganic cage. The positions of the NH3(+) stretching vibrations point to significant hydrogen bonding between the methylammonium and the halides for all three perovskites.

Spectroscopic Study of Anisotropic Excitons in Single Crystal Hexacene

The linear optical response of hexacene single crystals over a spectral range of 1.3-1.9 eV was studied using polarization-resolved reflectance spectroscopy at cryogenic temperatures. We observe strong polarization anisotropy for all optical transitions. Pronounced deviations from the single-molecule, solution-phase spectra are present, with a measured Davydov splitting of 180 meV, indicating strong intermolecular coupling. The energies and oscillator strengths of the relevant optical transitions and polarization-dependent absorption coefficients are extracted from quantitative analysis of the data.

Effect of Molecule-Surface Reaction Mechanism on the Electronic Characteristics and Photovoltaic Performance of Molecularly Modified Si

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 (S_N) and radical chain reaction (RCR), depending on adsorption conditions. Moderate heating leads to the S_N reaction, whereas with UV irradiation RCR dominates, with S_N as a secondary path. We show that the site-sensitive S_N reaction 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 S_N and 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.

Substituent variation drives metal/monolayer/semiconductor junctions from strongly rectifying to ohmic behavior

An eight-orders of magnitude enhancement in current across Hg/X-styrene-Si junctions is caused by merely altering a substituent, X. Interface states are passivated and, depending on X, the Si Schottky junction encompasses the full range from Ohmic to strongly rectifying. This powerful electrostatic molecular effect has immediate implications for interface band alignment and sensing.

Mono-fluorinated alkyne-derived SAMs on oxide-free Si(111) surfaces: preparation, characterization and tuning of the Si workfunction

Organic monolayers derived from ω-fluoro-1-alkynes of varying carbon chain lengths (C(10)-C(18)) were prepared on Si(111) surfaces, resulting in changes of the physical and electronic properties of the surface. Analysis of the monolayers using XPS, Infrared Reflection Absorption Spectroscopy, ellipsometry and static water contact angle measurements provided information regarding the monolayer thickness, the tilt angle, and the surface coverage. Additionally, PCFF molecular mechanics studies were used to obtain information on the optimal packing density and the layer thickness, which were compared to the experimentally found data. From the results, it can be concluded that the monolayers derived from longer chain lengths are more ordered, possess a lower tilt angle, and have a higher surface coverage than monolayers derived from shorter chains. We also demonstrate that by substitution of an H by F atom in the terminal group, it is possible to controllably modify the surface potential and energy barrier for charge transport in a full metal/monolayer-semiconductor (MOMS) junction.

Hybrids of organic molecules and flat, oxide-free silicon: high-density monolayers, electronic properties, and functionalization

Since the first report of Si-C bound organic monolayers on oxide-free Si almost two decades ago, a substantial amount of research has focused on studying the fundamental mechanical and electronic properties of these Si/molecule surfaces and interfaces. This feature article covers three closely related topics, including recent advances in achieving high-density organic monolayers (i.e., atomic coverage >55%) on oxide-free Si(111) substrates, an overview of progress in the fundamental understanding of the energetics and electronic properties of hybrid Si/molecule systems, and a brief summary of recent examples of subsequent functionalization on these high-density monolayers, which can significantly expand the range of applicability. Taken together, these topics provide an overview of the present status of this active area of research.

Molecules on si: electronics with chemistry

Basic scientific interest in using a semiconducting electrode in molecule-based electronics arises from the rich electrostatic landscape presented by semiconductor interfaces. Technological interest rests on the promise that combining existing semiconductor (primarily Si) electronics with (mostly organic) molecules will result in a whole that is larger than the sum of its parts. Such a hybrid approach appears presently particularly relevant for sensors and photovoltaics. Semiconductors, especially Si, present an important experimental test-bed for assessing electronic transport behavior of molecules, because they allow varying the critical interface energetics without, to a first approximation, altering the interfacial chemistry. To investigate semiconductor-molecule electronics we need reproducible, high-yield preparations of samples that allow reliable and reproducible data collection. Only in that way can we explore how the molecule/electrode interfaces affect or even dictate charge transport, which may then provide a basis for models with predictive power.To consider these issues and questions we will, in this Progress Report, review junctions based on direct bonding of molecules to oxide-free Si.describe the possible charge transport mechanisms across such interfaces and evaluate in how far they can be quantified.investigate to what extent imperfections in the monolayer are important for transport across the monolayer.revisit the concept of energy levels in such hybrid systems.

Molecular electronics at metal/semiconductor junctions. Si inversion by sub-nanometer molecular films

Electronic transport across n-Si-alkyl monolayer/Hg junctions is, at reverse and low forward bias, independent of alkyl chain length from 18 down to 1 or 2 carbons! This and further recent results indicate that electron transport is minority, rather than majority carrier dominated, occurs via generation and recombination, rather than (the earlier assumed) thermionic emission, and, as such, is rather insensitive to interface properties. The (m)ethyl results show that binding organic molecules directly to semiconductors provides semiconductor/metal interface control options, not accessible otherwise.

Interaction of Fe(III) tetrakis(4-N-methylpyridinium)porphyrin with sodium dodecyl sulfate at submicellar concentrations

Interaction of water soluble Fe(III) tetrakis(4- N-methylpyridinium)porphyrin (Fe(III)TMPyP) with sodium dodecyl sulfate (SDS) in submicellar concentrations has been studied by surface tension, optical absorption, resonance light scattering (RLS), zeta-potential, and energy dispersive X-ray spectroscopy (EDS) measurements. Measurements were conducted for a fixed concentration of Fe(III)TMPyP (6 x 10 (-5) M) and SDS in various concentrations ranging between 6 x 10 (-6) and 6 x 10 (-2) M. Two macroscopic phase transitions, precipitation and redissolution, were observed as function of SDS concentration. The presence of a new surface active porphyrin-surfactant complex was detected. Furthermore, the presence of two oppositely charged Fe(III)TMPyP-SDS bulk moieties has been demonstrated. Possible structures for the different moieties are suggested, and the phase transitions are discussed.