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Solvation plays a pivotal role in chemistry and biology. A solid-state analogy of solvation is polaron formation, but the magnitude of Coulomb screening is typically an order of magnitude weaker than that of solvation in aqueous solutions. Here, we describe a new class of polarons, the ferroelectric large polaron, proposed initially by Miyata and Zhu in 2018 (Miyata, K.; Zhu, X.-Y.Ferroelectric Large Polarons. Nat. Mater.2018, 17(5), 379−381). This type of polaron allows efficient Coulomb screening of an electron or hole by extended ordering of dipoles from symmetry-broken unit cells. The local ordering is reflected in the ferroelectric-like THz dielectric responses of lead halide perovskites (LHPs) and may be partially responsible for their exceptional optoelectronic performances. Despite the likely absence of long-range ferroelectricity in LHPs, a charge carrier may be localized to and/or induce the formation of nanoscale domain boundaries of locally ordered dipoles. Based on the known planar nature of energetically favorable domain boundaries in ferroelectric materials, we propose that a ferroelectric polaron localizes to planar boundaries of transient polar nanodomains. This proposal is supported by dynamic simulations showing sheet-like transient electron or hole wave functions in LHPs. Thus, the Belgian-waffle-shaped ferroelectric polaron in the three-dimensional LHP crystal structure is a large polaron in two dimensions and a small polaron in the perpendicular direction. The ferroelectric large polaron may form in other crystalline solids characterized by dynamic symmetry breaking and polar fluctuations. We suggest that the ability to form ferroelectric large polarons can be a general principle for the efficient screening of charge carriers from scattering with other charge carriers, with charged defects and with longitudinal optical phonons, thus contributing to enhanced optoelectronic properties.

The possibility of confining interlayer excitons in interfacial moiré patterns has recently gained attention as a strategy to form ordered arrays of zero-dimensional quantum emitters and topological superlattices in transition metal dichalcogenide heterostructures. Strain is expected to play an important role in the modulation of the moiré potential landscape, tuning the array of quantum dot-like zero-dimensional traps into parallel stripes of one-dimensional quantum wires. Here, we present real-space imaging of unstrained zero-dimensional and strain-induced one-dimensional moiré patterns along with photoluminescence measurements of the corresponding excitonic emission from WSe2/MoSe2heterobilayers. Whereas excitons in zero-dimensional moiré traps display quantum emitter-like sharp photoluminescence peaks with circular polarization, the photoluminescence emission from excitons in one-dimensional moiré potentials shows linear polarization and two orders of magnitude higher intensity. These results establish strain engineering as an effective method to tailor moiré potentials and their optoelectronic response on demand.

Semiconductor quantum dots (QDs) have been called artificial atoms because of their discrete electronic structures. Assembling them into artificial molecules may greatly expand our capability in controlling physical properties on the nanoscale. Here we show the successful assembly and size control of colloidal PbSe QD clusters into large-scale templates defined by block-copolymer patterns. Following the exchange of capping molecules, the QD clusters behave as artificial molecules due to enhanced and local electronic interactions.

Lead halide perovskites (LHPs) are attractive material systems for light emission, thanks to the ease and diverse routes of synthesis, the broad tunability in color, the high emission quantum efficiencies, and the strong light–matter coupling which may potentially lead to exciton-polariton condensation. This account contrasts the laser-like coherent light emission from highly lossy Fabry–Perot cavities, formed naturally from LHP nanowires (NWs) and nanoplates (NPs), with highly reflective cavities made of LHP gain media, sandwiched between two distributed Bragg reflector (DBR) mirrors. The mechanism responsible for the operation of conventional semiconductor lasers involves stimulated emission of electron and hole pairs bound by the Coulomb potential, i.e., excitons or, at excitation density above the so-called Mott threshold, an electron–hole plasma (EHP). We discuss how lasing from LHP NWs or NPs likely originates from stimulated emission of an EHP, not excitons or exciton-polaritons. A character central to this kind of lasing is the dynamically changing photonic properties in the naturally formed cavity. In contrast to the more static conditions of a DBR cavity, lasing modes and gain profiles are extremely sensitive to material properties and excitation conditions in an NW/NP cavity. While such unstable photonic cavities pose engineering challenges in the application of NW/NP lasers, they provide excellent probes of many-body physics in the LHP material.

Alloying inorganic cations such as Cs+or Rb+into hybrid lead halide perovskites has been shown to provide long-term material stability critical to the implementation in solar cells, but little is known about how cation alloying affects the electronic properties. Here we study single crystals of mixed-cation lead bromide perovskite, Csx(CH3NH3)1–xPbBr3, in a range of cation mixing ratios, x= 0.05–0.3. In contrast to the continuous bandgap tunability known for mixed-anion alloys, we find that the bandgaps of Csx(CH3NH3)1–xPbBr3single crystals adopt a bimodal distribution, with bandgaps similar to those of pure CH3NH3PbBr3for x≤ 0.13 and pure CsPbBr3for x> 0.13. Single-crystal X-ray diffraction reveals a structural origin of this abrupt change in bandgap: with increasing Cs concentration, there is a ∼3% lattice shrinkage and appearance of twin splitting at x≥ 0.13. These findings are manifestations of the structural complexity and phase instability of lead halide perovskites.

Structural phase transitions run in families of crystalline solids. Perovskites, for example, feature a remarkable number of structural transformations that produce a wealth of exotic behaviors, including ferroelectricity, magnetoresistance, metal–insulator transitions and superconductivity. In superatomic crystals and other such materials assembled from programmable building blocks, phase transitions offer pathways to new properties that are both tunable and switchable. Here we describe [Co6Te8(PEt3)6][C70]2, a novel superatomic crystal with two separate phase transitions that drastically transform the collective material properties. A coupled structural-electronic phase transition triggers the emergence of a new electronic band in the fullerene sublattice of the crystal, increasing its electrical conductivity by 2 orders of magnitude, while narrowing its optical gap and increasing its spin density. Independently, an order–disorder transition transforms [Co6Te8(PEt3)6][C70]2from a phonon crystal to a phonon glass. These results introduce a family of materials in which functional phase transformations may be manipulated by varying the constituent building blocks.

Mesenchymal stem cells (MSCs) constitute an important repair system, but may be impaired by exposure to cardiovascular risk factors. Consequently, adipose tissue-derived MSCs from pigs with the metabolic syndrome (MetS) show decreased vitality. A growing number of microRNAs (miRNAs) are recognized as key modulators of senescence, but their role in regulating senescence in MSC in MetS is unclear. We tested the hypothesis that MetS upregulates in MSC expression of miRNAs that can serve as post-transcriptional regulators of senescence-associated (SA) genes. MSCs were collected from swine abdominal adipose tissue after 16 weeks of Lean or Obese diet (n= 6 each). Next-generation miRNA sequencing (miRNA-seq) was performed to identify miRNAs up-or down-regulated in MetS-MSCs compared with Lean-MSCs. Functional pathways of SA genes targeted by miRNAs were analyzed using gene ontology. MSC senescence was evaluated by p16 and p21 immunoreactivity, H2AX protein expression, and SA-β-Galactosidase activity. In addition, gene expression of p16, p21, MAPK3 (ERK1) and MAPK14, and MSC migration were studied after inhibition of SA-miR-27b. Senescence biomarkers were significantly elevated in MetS-MSCs. We found seven upregulated miRNAs, including miR-27b, and three downregulated miRNAs in MetS-MSCs, which regulate 35 SA genes, particularly MAPK signaling. Inhibition of miR-27b in cultured MSCs downregulated p16 and MARP3 genes, and increased MSC migration. MetS modulates MSC expression of SA-miRNAs that may regulate their senescence, and the p16 pathway seems to play an important role in MetS-induced MSC senescence.

This Communication details the implementation of a new concept for the design of high-performance optoelectronic materials: three-dimensional (3D) graphene nanostructures. This general strategy is showcased through the synthesis of a three-bladed propeller nanostructure resulting from the coupling and fusion of a central triptycene hub and helical graphene nanoribbons. Importantly, these 3D graphene nanostructures show remarkable new properties that are distinct from the substituent parts. For example, the larger nanostructures show an enhancement in absorption and decreased contact resistance in optoelectronic devices. To show these enhanced properties in a device setting, the nanostructures were utilized as the electron-extracting layers in perovskite solar cells. The largest of these nanostructures achieved a PCE of 18.0%, which is one of the highest values reported for non-fullerene electron-extracting layers.

Lead halide perovskites (LHPs) are solution processable semiconductors characterized by long carrier lifetimes. Recent studies have suggested that electrons and holes in LHPs interact with phonons to form large polarons on subpicosecond time-scales and polaron formation may also slow down hot carrier cooling. Using femtosecond time-resolved two-photon photoemission (TR-2PPE) and transient reflectance (TR) spectroscopies, we follow the initial electron cooling and polaron formation dynamics in single-crystal CsPbBr3perovskite. We find that the hot electrons cool down initially (≤0.2 ps) with rates of −0.64 ± 0.06 eV/ps and −0.82 ± 0.08 eV/ps at 300 and 80 K, respectively. This weakly temperature-dependent rate is attributed to the initial relaxation of unscreened hot electrons by the emission of longitudinal optical (LO) phonons. On longer time scales, we observe dynamic changes in the photoemission cross-section and in the red-shift of the optical bandgap. We attribute these dynamic changes to large polaron formation from electron–LO phonon interaction, with temperature-dependent polaron formation time constants of τp= 0.7 ± 0.1 and 2.1 ± 0.2 ps at 300 and 80 K, respectively. The increase in polaron formation rate with temperature is correlated with the broadening in phonon resonances, suggesting that phonon disorder and dephasing facilitate large-polaron formation. The large polaron formation rate is not competitive with the cooling rate of unscreened hot electrons in CsPbBr3, in contrast to hybrid CH3NH3PbBr3(or CH3NH3PbI3) where the two rates are similar. This contrast explains the observation of long-lived hot carriers in the latter but not the former.

The controlled introduction of impurities into the crystal lattice of solid-state compounds is a cornerstone of materials science. Intercalation, the insertion of guest atoms, ions or molecules between the atomic layers of a host structure, can produce novel electronic, magnetic and optical properties in many materials. Here we describe an intercalation compound in which the host [Co6Te8(PnPr3)6][C60]3, formed from the binary assembly of atomically precise molecular clusters, is a superatomic analogue of traditional layered atomic compounds. We find that tetracyanoethylene (TCNE) can be inserted into the superstructure through a single-crystal-to-single-crystal transformation. Using electronic absorption spectroscopy, electrical transport measurements and electronic structure calculations, we demonstrate that the intercalation is driven by the exchange of charge between the host [Co6Te8(PnPr3)6][C60]3and the intercalant TCNE. These results show that intercalation is a powerful approach to manipulate the material properties of superatomic crystals.

The absorption of a photon usually creates a singlet exciton (S1) in molecular systems, but in some cases S1may split into two triplets (2×T1) in a process called singlet fission. Singlet fission is believed to proceed through the correlated triplet-pair 1(TT) state. Here, we probe the 1(TT) state in crystalline hexacene using time-resolved photoemission and transient absorption spectroscopies. We find a distinctive 1(TT) state, which decays to 2×T1with a time constant of 270 fs. However, the decay of S1and the formation of 1(TT) occur on different timescales of 180 fs and <50 fs, respectively. Theoretical analysis suggests that, in addition to an incoherent S1→1(TT) rate process responsible for the 180 fs timescale, S1may couple coherently to a vibronically excited 1(TT) on ultrafast timescales (<50 fs). The coexistence of coherent and incoherent singlet fission may also reconcile different experimental observations in other acenes.

Two issues related to the average period τcearly‐warning parameter are the magnitude saturation effect on large earthquakes and considerable scatter for small earthquakes. To reduce the effect of these two issues on earthquake early‐warning systems, we introduce a new τcregression relationship derived from all Pwave time windows (PTWs) in high‐pass filtered (T= 0.075 Hz) strong‐motion data for three damaging moderate‐to‐large earthquakes. Our results show that this relationship provides a better and more stable magnitude prediction than those derived from 3 s PTW without a saturation effect on large earthquakes with M< 7.5. It is expected that fewer false alerts (those outside the magnitude uncertainty tolerance) would be issued. Additionally, a reduction of the initial PTWs to 1–2 s and evolutionary calculation with an expanding window allow more lead time for small‐to‐moderate events. A new τcmagnitude relationship using all Pwave time windows without saturation for events with M< 7.5The relationship provides a better magnitude prediction than that derived from 3 s Pwave informationThe relationship is expected to improve EEWS performance with fewer false alarms, especially for small earthquakes

Few studies have presented a comparison of myeloablative cord blood transplantation (CBT) and HLA-identical sibling hematopoietic cell transplantation (HCT) for AML in a disease-specific analysis, and the evaluation of GvHD-free and relapse-free survival (GRFS) in AML patients after unrelated CBT has not been reported. A total of 162 consecutive AML patients receiving intensified myeloablative unrelated CBT (n=107) or allogeneic PBSC transplantation (allo-PBSCT) or bone marrow transplantation (BMT) from an HLA-identical sibling donor (n=55) were investigated. Neutrophil or platelet engraftment was slower in the CBT cohort compared with that in the allo-PBSCT/BMT cohort. The incidence of grade II–IV or grade III–IV acute GvHD (aGvHD) and transplant-related mortality (TRM) were not significantly different in the two cohorts. Compared with the allo-PBSCT/BMT cohort, the CBT cohort had a significantly lower rate of chronic GvHD (cGvHD) (13.7% vs 28.3%; P=0.047) or extensive cGvHD (9.9% vs 24.1%; hazard ratio (HR)=2.06, P=0.039). The incidence of relapse at 5 years in the CBT cohort was significantly lower than that in the allo-PBSCT/BMT cohort (15.3% vs 36.1%; HR=4.62, P=0.009). The probabilities of overall survival and leukemia-free survival were similar between the two cohorts. The adjusted 5-year probability of GRFS was higher after CBT than that after allo-PBSCT/BMT (55.4% vs 39.2%; HR=1.63, P=0.042). The present study suggests that, for AML patients, intensified myeloablative unrelated CBT is associated with less cGvHD and a lower risk of relapse. In addition, these patients do not experience excessive TRM or severe aGvHD that translates into better GRFS compared with those patients who undergo HLA-identical sibling allo-PBSCT/BMT; this observation may reflect the clinical separation between cGvHD and GvL within our CBT protocol.

Interest in materials that undergo singlet fission (SF) has been catalyzed by the potential to exceed the Shockley–Queisser limit of solar power conversion efficiency. In conventional materials, the mechanism of SF is an intermolecular process (xSF), which is mediated by charge transfer (CT) states and depends sensitively on crystal packing or molecular collisions. In contrast, recently reported covalently coupled pentacenes yield ∼2 triplets per photon absorbed in individual molecules: the hallmark of intramolecular singlet fission (iSF). However, the mechanism of iSF is unclear. Here, using multireference electronic structure calculations and transient absorption spectroscopy, we establish that iSF can occur via a direct coupling mechanism that is independent of CT states. We show that a near-degeneracy in electronic state energies induced by vibronic coupling to intramolecular modes of the covalent dimer allows for strong mixing between the correlated triplet pair state and the local excitonic state, despite weak direct coupling.

Kiyoshi Miyata and X.-Y. Zhu analyse the ferroelectric-like dielectric response of lead halide perovskites in the terahertz region and discuss the potential role of polar nanodomains in accounting for the defect tolerance and low recombination rates of these materials.

Ultra low frequency (ULF) waves are fundamental waves that can energize, transport, and scatter charged particles in planetary magnetospheres. With the measurements from MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER), we investigate the proton flux fluctuations and coherent waves associated with a series of ULF waves on the flanks of Mercury's magnetosphere. The ULF waves are mainly compressional with a frequency of ∼15 mHz and significantly modulate the intensity of proton flux. The coherent waves accompanied by the ULF waves correspond to a higher frequency (∼1 Hz). The wave power and compressibility of the coherent waves vary quasi‐periodically with the ∼15 mHz ULF waves. We conclude that the compressional ULF waves modulate the coherent waves with higher frequency. This modulation might result from the associated periodic proton flux changes and helps us understand the nature of the ∼1 Hz waves better. This study presents MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) observations of wave‐particle interactions on the flank of Mercury's magnetosphere. Clear one‐to‐one modulation of proton flux by ∼15 mHz compressional waves is shown in these two cases. Proton gyro‐frequency (∼1 Hz) waves are observed during the same interval. Their amplitudes and compressibilities are periodic with a frequency of ∼15 mHz, coinciding with the frequency of the compressional waves. This coincidence indicates that the ∼1 Hz waves are possibly affected by the ∼15 mHz waves. The above observational facts demonstrate that ULF waves can modify the plasma environment and further affect the kinetic scale dynamics in the magnetosphere of Mercury. Its importance is further validated by our statistical result, which shows that wave‐particle and wave‐wave modulation are not rare events. ∼15 mHz magnetic field pulsations were detected and found to be capable of driving proton flux fluctuations in Mercury's magnetosphereThe associated ∼1 Hz magnetic waves have ∼15 mHz variations in power and compressibility, indicating modulation by the ∼15 mHz wavesThe influence on ∼1 Hz waves is possibly attained by plasma flux changes, helping us better interpret its wave mode and character ∼15 mHz magnetic field pulsations were detected and found to be capable of driving proton flux fluctuations in Mercury's magnetosphere The associated ∼1 Hz magnetic waves have ∼15 mHz variations in power and compressibility, indicating modulation by the ∼15 mHz waves The influence on ∼1 Hz waves is possibly attained by plasma flux changes, helping us better interpret its wave mode and character

The size-dependent optical gap due to the quantum confinement effect is a hallmark of semiconductor quantum dots (QDs). A major research effort underway is to control energy transfer in QDs for diverse applications ranging from solar energy harvesting to fluorescent displays and biological imaging. In this work, we report an innovative approach for fabricating monodispersed arrays of Au/polymer/QDs sandwiched nanodisks on a wafer scale with high rationality and reproducibility. Via template-assisted assembling of QDs, the plasmonic Au nanodisk can be precisely coupled to a defined number of QD clusters. Using steady-state and time-resolved photoluminescence spectroscopy, we studied the enhancement of Förster resonance energy transfer (FRET) rate between donor and acceptor QDs coupled with plasmonic resonance of the Au nanodisks. By systematically increasing the spectral overlap of the acceptor QD emission with plasmonic resonance, we demonstrated that the donor lifetime decreases monotonically and FRET rate increases significantly. The results suggest a viable route to fabricate precisely controlled plasmonic–QD coupled nanoentities and to enhance energy transfer at the nanoscale.

While the perovskite fever has focused on three-dimensional crystalline solids, this class of material can also self-assemble into two-dimensional (2D) layered structures that are natural quantum wells with tunable thickness and optoelectronic properties. Here we apply femtosecond transient absorption spectroscopy to study the many-body optical responses of 2D perovskites with the general formula of (C4H9NH3I)2(CH3NH3I)n−1(PbI2)n, where n= 1, 2, 3) is the number of lead iodide unit cells in the direction perpendicular to the 2D quantum well. In the thinnest quantum well (n= 1), above-gap optical excitation induces a blue shift but no population bleaching at the excitonic resonance; this is similar to the many-body optical response of conventional inorganic quantum wells. In contrast to inorganic quantum wells, we find the excitonic blue-shift in 2D perovskites to be independent of excitation power density. We take this as evidence for a Mott-Wannier exciton localizing into a “puddle”, which only exerts local influence on subsequent optical excitations. The excitonic puddles likely come from the disordered electronic energy landscape expected for the soft 2D hybrid organic–inorganic perovskite lattice. As the thickness of the quantum well increases to n= 3, free carrier characters start to show up for above band gap excitation; this is reflected in the broadening and bleaching of the excitonic resonance (in addition to blue-shift), attributed to carrier-exciton collision and screening of the Coulomb potential, respectively.

Interfacial charge transfer is ubiquitous in many chemical and physical processes and can occur on ultrafast time scales of femtoseconds to picoseconds. Probing dynamics on such time scales necessitates the use of ultrafast laser spectroscopies, but signatures of interfacial charge transfer can be overwhelmed by the signal from bulk materials. This problem may be alleviated in second-harmonic generation, which can be specifically sensitive to interfacial charge transfer if other bulk and interfacial contributions to the measured second-harmonic signal can be resolved. We report the development of a femtosecond spectral interferometry technique for second-harmonic generation with time, energy, and phase resolution. Using the model systems of a passivated GaAs(100) surface and copper phthalocyanine/GaAs(100) interface, we demonstrate the application of this technique in unveiling the rich dynamics of band renormalization, charge carrier motion, and interfacial charge transfer, all induced by across-bandgap optical excitation of the semiconductor.

The binding of lectins to glycan receptors on the host cell surface is a key step contributing to the virulence and species specificity of most viruses. This is exemplified by the viral protein hemagglutinin (HA) of the influenza A virus, whose binding specificity is modulated by the linkage pattern of terminal sialic acids on glycan receptors of host epithelial cells. Such specificity dictates whether transmission is confined to a particular animal species or jumps between species. Here, we show, using H5N1 avian influenza as a model, that the specific binding of recombinant HA to α2-3 linked sialic acids can be enhanced dramatically by interaction with the surface of the lipid membrane. This effect can be quantitatively accounted for by a two-stage process in which weak association of HA with the membrane surface precedes more specific and tighter binding to the glycan receptor. The weak protein–membrane interaction discovered here in the model system may play an important secondary role in the infection and pathogenesis of the influenza A virus.

We have probed photoinduced charge separation dynamics in organic semiconductor thin films and at their interfaces by femtosecond time-resolved second harmonic generation (TR-SHG), using the model systems of fullerene (C70) thin films and copper phthalocyanine (CuPc)/C70interfaces. In neat C70thin films, the formation of an internal electric field on a ∼10-ps time scale following photoexcitation is attributed to the photo-Dember effect, namely, charge separation resulting from a gradient in excitation density and the differential electron/hole mobility. When an ultrathin film of the electron donor CuPc was deposited on top of the C70film, an additional interfacial charge separation channel occurring on the ultrafast time scale of ∼0.1 ps was observed. We discuss how SHG fields from different origins can interfere to give the overall transient response.

Organic/inorganic semiconductor heterojunctions are being explored in hybrid solar cells that take advantage of unique properties of both material systems. A key question concerns the mechanism of charge separation across the localized/delocalized semiconductor interface. Here we probe photoinduced charge transfer at a model interface between copper phthalocyanine (CuPc) and gallium arsenide (GaAs) by tracking the electric field on the femtosecond time scale using time-resolved second harmonic generation. For above bandgap excitation of GaAs, we resolve distinction channels of charge separation on 102fs time scales: charge carrier separation in GaAs due to the delocalized space charge field and hole injection from photoexcited GaAs to localized CuPc molecular orbitals. At sufficiently high excitation density (>1017/cm3), charge separation by the space charge field leads to band flattening, which accelerates resonant hole transfer from GaAs to CuPc. We discuss implications of these findings to the design of organic/inorganic hybrid solar cells.

This paper deals with mechanical systems with passive contact elements, such as whole-hand grasps and fixture systems. The force closure problem in fixtures and manipulation systems with hybrid (active and passive) contacts is shown to be quite different from the pure active force closure in a multi-fingered robot hand. We discuss the conditions of active and passive force closure problems in such devices based on a rigid-body model with frictional contact. The contact force space is decomposed into three orthogonal subspaces: the space of external forces, the space of active internal forces, and the space of passive internal forces. The space of active internal force and the space of passive internal force play important roles in achieving the closure grasping. This paper presents an algorithm for computing the basis of each subspace. Based on this force decomposition, the force closure problem is formulated as linear matrix inequality feasibility problems, and minimization of the joint torque is also studied.

We report comprehensive characterization of electrolyte-gated polymer thin-film transistors (TFTs) incorporating solution processable polymer semiconductors and high capacitance “ion gel” gate dielectrics. The ion gel dielectrics comprise self-assembled networks of triblock copolymers such as poly(styrene-b-methylmethacrylate-b-styrene) [PS-PMMA-PS] that are swollen with ionic liquids, e.g., (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]). The capacitance of the gels is exceptionally large (>10 μF/cm2at 10 Hz), which is derived from the high concentration of mobile ions and facilitates operation of ion gel-gated organic TFTs (GEL-OTFTs) at very low voltages (< 2.5 V). Gate-induced hole densities in GEL-OTFTs employing different polythiophene semiconductors in the channel are on the order of 1014carriers/cm2, with associated saturation hole mobilities that are also remarkably large, ∼1 cm2/(V s), likely because of the large gate-induced carrier densities. Examination of the frequency response of GEL-OTFTs indicates that increases in the OFF current with frequency ultimately limit switching speed; the cutoff frequency correlates with the ionic conductivity versus frequency response of the gel dielectric. Further, attenuated total internal reflection infrared (ATR-IR) spectroscopy of the ion gel/polymer semiconductor gate stack reveals that the conductance switching mechanism in GEL-OTFTs spans both electrochemical and electrostatic (field effect) regimes. Specifically, modeling of the time dependence of the near-infrared polaron absorption in gated GEL-OTFTs indicates that the [TFSI]−anion diffusivity in regioregular poly(3-hexylthiophene) is on the order of 10−12cm2/s at room temperature. This diffusivity implies that, for time scales greater than 1 ms, there is significant penetration (>1 nm) of [TFSI]−anion into the polymer semiconductor at the gel/polymer semiconductor interface, corresponding to an electrochemical doping process. On the other hand, for time scales shorter than 1 ms (i.e., for GEL-OTFT switching frequencies >1 kHz), the device switching mechanism can be viewed as primarily electrostatic as average ion penetration depths are less than 1 nm.

Thin films of colloidal PbSe quantum dots can exhibit very high carrier mobilities when the surface ligands are removed or replaced by small molecules, such as hydrazine. Charge transport in such films is governed by the electronic exchange coupling energy (β) between quantum dots. Here we show that two-dimensional quantum dot arrays assembled on a surface provide a powerful system for studying this electronic coupling. We combine optical spectroscopy with atomic force microscopy to examine the chemical, structural, and electronic changes that occur when a submonolayer of PbSe QDs is exposed to hydrazine. We find that this treatment leads to strong and tunable electronic coupling, with the β value as large as 13 meV, which is 1 order of magnitude greater than that previously achieved in 3D QD solids with the same chemical treatment. We attribute this much enhanced electronic coupling to reduced geometric frustration in 2D films. The strongly coupled quantum dot assemblies serve as both charge and energy sinks. The existence of such coupling has serious implications for electronic devices, such as photovoltaic cells, that utilize quantum dots.

Stabilization of excess electrons is studied at crystalline ice−metal interfaces by femtosecond time-resolved two-photon photoelectron spectroscopy and ab initio calculations. Following optical excitation into delocalized image potential states, electrons localize at pre-existing defects which are located at the ice−vacuum interface. Remarkably, the stabilization of these trapped electrons is monitored continuously from femtoseconds up to minutes. By first principle calculations we identify defect structures, that support excess electrons in front of crystalline ice surfaces, and suggest that the stabilization proceeds through subsequent conformational substates. The excess electron wave functions are efficiently screened from the underlying bulk and explain the long residence times observed in the experiment. Thereby, we shed light on the collective character of the nuclear rearrangement that determines the energetics over all timescales.

We carried out in situ optical spectroscopy measurements of gate-doped poly(3-hexylthiophene) (P3HT) using a polymer electrolyte dielectric. At low doping levels, hole polarons are present in two distinct environments: crystalline and amorphous phases of P3HT. As carrier concentration increases to a region corresponding to the insulator-to-metal transition in transistor measurements, the two polaron states merge into a single state. We take this spectroscopic signature as evidence for strong carrier screening which removes the energetic barrier for polaron transfer from crystalline to amorphous domains; this is responsible for the insulator-to-metal transition in highly doped P3HT.

We use femtosecond time-resolved two-photon photoemission spectroscopy (TR-2PPE) to study the dynamics of electrons excited at the ZnO (101̅0) surface. Efficient relaxation of hot electrons within the Γ valley of the bulk conduction band results in sub-30 fs lifetimes for electron energies greater than 0.1 eV above the conduction band minimum (CBM). These relaxation rates, which are among the fastest observed in any semiconductor over the same energy range, are consistent with the emission of longitudinal optical phonons resulting from strong Fröhlich coupling. For energy at or below the CBM, the excited electron lifetime increases exponentially with decreasing energy to as long as 1 ps. Dynamics in this region can be described by electronic relaxation within a quasi-continuum of defect-derived surface states whose density decreases exponentially into the band gap. Deliberately increasing defects on the ZnO surface drastically decreases the lifetime of electrons in this energy region. Existence of these states is consistent with observed upward band-bending and Fermi level pinning at the (101̅0) surface.

A key issue governing efficient electron transfer between two semiconductors is interfacial electronic energy alignment. We address this issue in a model system relevant to quantum-dot-sensitized solar cells, cadmium selenide (CdSe) quantum dots adsorbed on a single crystal zinc oxide (ZnO) (101̅0) surface via 3-mercaptopropionic acid linkers, using ultraviolet photoelectron spectroscopy. The valence band maximum (VBM) of the CdSe quantum dots is found to be located at 1.1 ± 0.1 eV above the VBM of ZnO, nearly independent of the size of the quantum dots (2.1−4.2 nm). This finding suggests that, upon adsorption, there is direct electronic interaction between CdSe quantum dots and the ZnO surface involving CdSe valance bands. Such electronic interaction pins the CdSe valence band to the Fermi level. As a result, varying the quantum dot size mainly tunes the alignment of the conduction band minimum of CdSe with respect to that of the ZnO surface.

We report the electrical transport behavior of a series of redox-active conjugated molecular wires as a function of temperature and molecular length. The wires consist of covalently coupled ruthenium(II) bis(-arylacetylide) complexes (Ru1−Ru3) and range in length from 2.4 to 4.9 nm. The molecules are unique in that they contain multiple metal-redox centers that are well-coupled by conjugated ligands. The molecules were self-assembled and their films were extensively characterized using ellipsometry, X-ray photoelectron spectroscopy, reflection−absorption infrared spectroscopy, and cyclic voltammetry. We probed their electrical properties using conducting probe atomic force microscopy and crossed-wire junctions. At room temperature, we found a very weak dependence of the wire resistance with molecular length, consistent with a high degree of electronic communication along the molecular backbone. In low-temperature (5 K) experiments, Coulomb blockade-like behavior was observed in junctions incorporating Ru3; direct tunneling appears to be the dominant transport mechanism in Ru1and Ru2junctions.

The formation of alkanethiol (H−(CH2)n−SH, n = 8−18) and 1,8-octanedithiol (HS−(CH2)8−SH) monolayer films on n-type GaAs(001) has been systematically studied. We observed a nonlinear dependence of the film thickness on molecular length, which is drastically different from monolayer films of the same molecules on metals. For 8 ≤ n ≤ 14, the films are only 3−4.5 Å thick, significantly smaller than the corresponding molecular length. For n = 16 and 18, the measured film thicknesses were 9 and 11 Å, respectively, consistent with molecules orienting with a tilt angle of ∼60° from the surface normal. Unlike the alkanethiols, the thickness of the 1,8-octanedithiol monolayer is almost the same as its molecular length, indicating that dithiol molecules orient vertically with only one thiol end group bound to the GaAs surface. Additional support for this conclusion comes from the fact that X-ray photoelectron spectroscopy of the 1,8-octanedithiol monolayer clearly resolves two types of S atoms in the monolayer:  those bound to the GaAs surface and those existing as free thiols. A suggestion was made on the mechanisms for alkanethiol and alkanedithiol monolayer formation.

Pentacene, as well as other polyacenes, is known to adsorb on metal surfaces in a lying-down geometry, that is, with the molecular plane parallel to the surface. Here we show using scanning tunneling microscopy that the lying-down pentacene monolayer seeds the growth of a multilayer polycrystalline film in a layer-by-layer fashion, with the molecular planes parallel to the substrate surface. Growth in the submonolayer region is characterized by a large number of ordered molecular structures as coverage increases. The saturated monolayer structure remains approximately the same throughout multilayer coverages studied. Statistical analysis of molecular domain orientation reveals that the ordered pentacene structures are incommensurate with the Au(111) substrate and growth beyond the monolayer region is a mixture of epitaxy and nonepitaxy. We conclude that the relatively large footprint of a lying-down pentacene molecule smoothes out lateral corrugation in the adsorption energy landscape on the Au(111) surface, leading to the growth of an incommensurate, polycrystalline, and partially nonepitaxial thin film.

Electrostatic interaction is known to play important roles in the adsorption of charged lipids on oppositely charged surfaces. Here we show that, even for charge neutral (zwitterionic) lipids, electrostatic interaction is critical in controlling the adsorption and fusion of lipid vesicles to form supported phospholipid bilayers (SPBs) on surfaces. We use terminally functionalized alkanethiol self-assembled monolayers (SAMs) to systematically control the surface charge density. Charge neutral egg phophatidylcholine (eggPC) vesicles readily fuse into SPBs on either a positively charged 11-aminino-1-undecanethiol SAM or a negatively charged 10-carboxy-1-decanethiol SAM when the density of surface charge groups is ≥80%. These processes depend critically on the buffer environment: fusion of adsorbed vesicles to form SPBs on each charged molecular surface does not occur when the molecular ion of the buffer used is of the opposite charge type. We attribute this to the high entropic repulsion (electric double layer repulsion) due to the large size of molecular counterions. On the other hand, such a critical dependence on buffer type is not observed when charged lipids are used. This study suggests the general importance of controlling electrostatic interaction in the formation of stable SPBs.

The tribological properties of alkoxyl monolayers on oxide terminated silicon surfaces have been investigated using interfacial force microscopy. For a C18 alkoxyl monolayer, both adhesion and frictional properties are similar to those of a self-assembled monolayer of octadecanethiol on gold. Friction is shown to increase as the alkyl chain length of the molecules decreases. Analysis using contact mechanics models has been carried out to estimate reduced modulus, adhesion energy, and friction shear strength. These interfacial mechanical properties are correlated to molecular structures at the interface.

Unoccupied electronic states in C60 thin films on Cu(111) have been measured by laser two-photon photoemission (2PPE) spectroscopy. These measurements allow the quantitative determination of energetic positions for the lowest unoccupied molecular orbital (LUMO), LUMO + 1, LUMO + 2, image potential states, as well as the highest occupied molecular orbital (HOMO). The transiently populated LUMO and LUMO + 1 levels are stabilized by the on-molecule charge correlation energy during the 2PPE process. Compared to previous measurements using one-photon photoemission and inverse photoemission, the HOMO−LUMO gap determined by 2PPE is stabilized by half of the Hubbard U. Both intramolecular excitation and a minor metal-to-molecule electron-transfer excitation channel are observed.

A strategy has been developed for the grafting of poly(ethylene glycol) (PEG) monolayers onto the atomically flat Si(111) surface. This process involves the direct interaction of the HO-functional group on PEG with a chlorine-terminated silicon surface. The resulting monolayers show exceptional flatness, with mean roughness on the angstrom scale. Film thickness is determined by ellipsometry, X-ray photoelectron spectroscopy, and atomic force microscopy. Results for different molecular weights reveal that the grafted PEG molecules are in a brushlike configuration at low molecular weight (MW = 200, 300) and a more coil-like configuration at higher molecular weight (MW = 1000, 2000). The surface density of grafted PEG molecules is significantly higher than those reported in the literature and decreases with increasing molecular weight, from 92% at MW = 300 to 35% at MW = 2000. Both the brushlike and the coil-like configurations show excellent properties of protein resistance.

We report evidence for an excited molecular anionic resonance in a model system, bilayer C6F6 adsorbed on Cu(111). This resonance is dispersed parallel to the surface, with an effective electron mass of 1.5 me. Both the energetic position and the dispersion of this molecular resonance depend on the strength of molecule−surface interaction, which is weakened by the preadsorption of atomic H. With increasing coverage of preadsorbed H from θ = 0 to 0.34 monolayer (ML), the position of the molecular resonance increases from 2.99 to 3.22 eV above the Fermi level. This is accompanied by a decrease in effective electron mass from 1.5 me on clean Cu(111) to 0.55 me on 0.34 ML H-covered Cu(111). We attribute this to the improved molecule−molecule interaction as the molecule−surface interaction weakens.

A new synthetic strategy has been developed for the formation of densely packed monolayers on oxide surfaces. This technique involves a simple two-step procedure in which the oxide surface is initially chlorinated and then exposed to alcohol molecules for monolayer formation via Si−O linkages. We demonstrate this approach with the formation of octadecanol (C18) monolayers on the oxide-terminated Si(111) surface. Characterization of the resulting C18H37O− monolayer by a combination of techniques reveals a densely packed film with partial disorder. At the highest surface coverage, the thickness of the film is determined to be 24 Å. Kinetics of monolayer growth is characterized by a fast initial step at low coverage; the rate constant decreases by 3 orders of magnitude when surface coverage approaches saturation.

Using interfacial force microscopy (IFM), the tribological properties of self-assembled monolayers (SAM) on Si surfaces produced by a new chemical strategy are investigated and compared to those of “classical” SAM systems, which include alkanethiols on Au and alkylsilanes on SiOx. The new SAM films are prepared by depositing n-alkyl chains with OH-terminations onto Cl-terminated Si substrates. The chemical nature of the actual lubricating molecules, n-dodecyl, is kept constant in all three thin film systems for direct comparison and similarities and differences in tribological properties are observed. The adhesion strength is virtually identical for all three systems; however, frictional properties differ due to differences in film packing. Differences in the chemical bonds that attach the lubricant molecules to the substrate are also discussed as they influence variations in film wear and durability. It is demonstrated that the new SAM films are capable of controlling the friction and adhesion of Si surfaces equally well as the classical SAMs and are potentially more reproducible and more durable.

Electron transfer at organic/metal interfaces is a fundamental issue of interest to a large number of problems in chemistry. We use two-photon photoemission (2PPE) spectroscopy to investigate heterogeneous electron transport in a model system:  naphthalene adsorbed on Cu(111). The dependence of 2PPE spectra on photon energy establishes the occurance of photoinduced electron transfer to an unoccupied state at 3.1 eV above the Fermi level (or 1.1 eV below the vacuum level) at one monolayer coverage. Polarization and dispersion measurements reveal the image-like property of this electron-transfer state. The binding energy of this state is dependent on the thickness of the adsorbate layer. This observation is in agreement with simulation based on the dielectric continuum model. The simulation also shows that, while partially distributed in the adsorbate layer at one monolayer coverage, the excited electron is confined within the adsorbate layer at high coverages, an effect which can be attributed to the electron affinity of naphthalene.

Monolayer assemblies on silicon surfaces are of interest for a number of technological reasons. Here, we present a novel, two-step strategy for assembly formation directly on silicon surfaces. In the first step, a H-terminated Si(100) or Si(111) surface reacts with Cl2 to give a Cl-capped surface. In the second step, the Cl−Si surface is immersed in an alcohol/isooctane solution for monolayer formation via Si−O linkages, with the removal of surface Cl likely in the form of HCl. This reaction mechanism is confirmed by X-ray photoelectron spectroscopy. X-ray reflectivity measurement shows that the thickness of the monolayer film is close to molecular dimension and the density is about 85% of that in crystalline paraffine. Fourier transform infrared spectroscopy and water contact angle measurements suggest that the alkyl chains within the monolayer possess, to a limited degree, conformational order. Atomic force microscopy image with molecular lattice resolution shows intermolecular distance corresponding to close packing of alkyl chains. The monolayer coatings show both chemical and mechanical stability. The possible mechanism for monolayer assembly formation on covalent Si surfaces is discussed.

Soft thin films, i.e., organic monolayers, are assembled on Si(100) from the chemical vapor deposition (CVD) of amine molecules (R−NH2) on the monochloride-covered surface. The N anchor is believed to be bridged between two surface Si atoms while the hydrocarbon group remains intact. Thermal stability of the resulting organic monolayer depends on molecular structure. In the phenyl assembly, which forms close packing on the Si(100) surface, the molecular monolayer is thermally stable; desorption and dissociation only occur at temperatures above 750 K. Alkyl assemblies are less stable; thermal desorption of hydrocarbons occurs below 700 K. The method demonstrated here is sufficiently general and can be extended to other CVD reactions for the covalent assembly on silicon surfaces in either vacuum environment or solution phases.

Microelectromechanical systems (MEMS) are poised to bring the next technology revolution. At present, many of these are fabricated from silicon using lithographic techniques developed in the microelectronics industry. Due to the large surface area to volume ratio on the micrometer scale, surface forces, such as adhesion and friction, are often detrimental to the fabrication and operation of MEMS devices. Thus, one of the key issues in MEMS is surface engineering to reduce adhesion and friction. Here, we present a general strategy for the efficient assembly of organic layers directly onto the silicon surface in both vacuum environment and solution phases via the robust Si–N or Si–O linkage. This is achieved by the reaction between an amine or alcohol functional group and a chlorinated Si surface. The resulting surface assemblies are thermally stable. Characterization by interfacial force microscope (IFM) reveals that these assemblies have very low surface energy and are mechanically stable.

Using interfacial force microscopy (IFM), we investigated the tribological behavior of hexadecanethiol monolayers on Au and films of octadecyltrichlorosilane (ODTS), perfluorodecyltrichlorosilane (PFTS) and dodecane on Si. We observe a strong correlation between hysteresis in a compression cycle (measured via nanoindentation) and friction. Additionally, we suggest that the amount of hysteresis and friction in each film is related to its detailed molecular structure, especially the degree of molecular packing.

Self-assembled-monolayers (SAMs) of aromatic thiolates on metal surfaces are candidates for molecular “quantum dots” or “wires”. We probe the interfacial electronic structure using two-photon photoemission in SAMs of pentafluorothiophenolate (C6F5S^-) on Cu(111). Compared to gas-phase C6F5SH, the σ* LUMO in the assembled molecule is stabilized by 3.1 eV and the HOMO−LUMO gap is reduced by 5.7 eV. Ab initio calculations reveal that stabilization of molecular states is, to a great extent, a consequence of the strong electronic coupling between metal and molecular orbitals via the −S− bridge. This strong electronic coupling is absent when the molecule is not directly bonded to the surface, as in the case of C6F6 weakly adsorbed on Cu(111).

Atomic hydrogen photodesorbs from the Si(100)−(2 × 1):H monohydride surface at 157 nm, with a cross section of ~3 × 10^-21 cm². We determine using polarized light that the transition dipole moment for the optical excitation is oriented at ~18° from the surface normal, as expected for the σ → σ* transition within the H−Si bond. This result unambiguously establishes the dissociation of the H−Si surface bond by direct optical excitation. The dynamics of this process is characterized by a translational energy of 〈Etrans〉 = 0.24 ± 0.02 eV for atomic H and an isotope effect in photodesorption yield of yH/yD = 10 ± 3. We carry out time-dependent quantum mechanical simulation on ab initio potential energy surfaces. The simulation suggests a subfemtosecond lifetime in the excited state and a quantum yield for atomic hydrogen desorption of the order 10^-5. This study provides an extreme example for bond-selective photochemistry in large chemical systems. In addition to its significance in understanding ultrafast surface dynamic processes, the direct photodesorption process demonstrated here may also find applications as resistless photolithography in semiconductor technology.

Photoinduced heterogeneous electron transfer (ET) constitutes the key step in a number of chemical processes, such as solar energy conversion, photocatalysis, and surface photochemistry. We present a general strategy to investigate photoinduced heterogeneous ET using two-photon photoemission (2PPE) spectroscopy. In this report, we focus on the energetics in photoinduced heterogeneous ET in monolayer and bilayers of hexafluorobenzene on Cu(111). We found that for monolayer C6F6, which likely adsorbs flat on the surface, the ET resonance is not visible in 2PPE spectrum. However, for second-layer C6F6, a strong molecular resonance is observed. Photon energy dependence shows that the shift in electron kinetic energy (ΔEkin) from the molecular resonance scales with Δhν, unlike that from the occupied surface state, which scales with 2Δhν. This provides evidence that the observed molecular resonance results from electron transfer to an unoccupied molecular orbital located at 2.9 eV above the Fermi level (or 1.8 eV below the vacuum level). We attribute this ET resonance to the occupation of the σ* LUMO level. Compared to the condensed phase, this negative ion state is lowered by 0.7 eV on the metal surface because of charge−image interaction.