Browse Articles

Growth of crystalline thin films of picene on semimetallic Bi(111) surface

We report the scanning tunneling microscopy/spectroscopy (STM/STS) studies on structural and electronic properties of picene films grown on the semimetallic Bi(111) substrate held at different temperatures. Under room-temperature deposition, the picene molecules form a crystalline (001) monolayer with the standing-up orientation, indicating the weak molecule–substrate interaction. When deposited on the Bi(111) substrate held at 150 K, picene molecules form a bulk-like (211̄ monolayer with building blocks of picene trimers. High-resolution STM images reveal that each trimer consists of two tilted molecules and one side-on molecule. Further reducing the deposition temperature to 90 K leads to the formation of nanostripe arrays, in which the side-on molecules adopt the π–π stacking. STS measurements demonstrate that the crystalline (001) monolayer of picene exhibits a larger gap compared with picene crystals, which can be attributed to the decoupling of the upright standing molecules from the semimetallic Bi(111) substrate.

Scaling law for the critical voltage of a droplet on a surface in the presence of an external field

Using external fields to regulate the droplet shape and behavior has attracted considerable attention due to its wide range of practical applications. However, determining the complex flow phenomenon inside a droplet and its deformable boundary shape at equilibrium is a challenging physical and mathematical exercise. In the present work, we employ theoretical and experimental approaches to study the shape deformation of a sessile droplet on a substrate in the presence of an external electric field. Based on the theoretical model we propose, and by combining the finite element method with the gradient descent algorithm, we successfully determine the droplet shape by minimizing the total free energy of the system, viz., a combination of electrostatic energy, surface tension energy, and gravitational potential energy. We also perform scaling analyses and derive an empirical expression for the critical voltage, featuring a universal scaling exponent of 1/2 for the contact angle as a function of normalized volume. The master curve depicted by the empirical expression provides an excellent fit to both the experimental and numerical results.

Study of the electronic structure of NixTiSe2 by EMF method

The electron structure of the NixTiSe2 system has been studied by the electromotive force (EMF) method in Cu|Cu+|NixTiSe2 and Na|Na+|NixTiSe2 electrochemical cells. Two critical transitions have been found in this system at x = 0.25 and x = 0.35. At these compositions, a change in the nature of the chemical bond of nickel with its local environment occurs. The simple and useful EMF method has been applied as the instrument to study features of electron structure. A comparative analysis of critical points in the electronic structure of the MexTiSe2 (Me = Fe, Co, Ni) intercalate compounds was carried out.

Optimal control design strategies for pulsed dynamic nuclear polarization

We present optimal control methods for the optimization of periodic pulsed dynamic nuclear polarization (DNP) sequences. Specifically, we address the challenge of the optimization of a basic and repeated pulse sequence element which, apart from being easily adaptable to spin systems with different coupling interaction sizes, also proves beneficial in terms of performance. It is demonstrated that matrix power and matrix logarithm functions combined with an auxiliary matrix formalism can be used to derive expressions for gradient ascent pulse engineering (GRAPE) optimization. We illustrate how different implementations provide effective and intuitive control of DNP experiments by tailoring the effective Hamiltonian governing polarization transfer and, in this manner, addressing some of the limitations of prevailing optimal control based pulse design strategies.

Repartitioning the Hamiltonian in many-body second-order Brillouin–Wigner perturbation theory: Uncovering new size-consistent models

Second-order Møller–Plesset perturbation theory is well-known as a computationally inexpensive approach to the electron correlation problem that is size-consistent with a size-consistent reference but fails to be regular. On the other hand, the less well-known many-body version of Brillouin–Wigner perturbation theory has the reverse properties: it is regular but fails to be size-consistent when used with the standard MP partitioning. Consequently, its widespread use remains limited. In this work, we analyze the ways in which it is possible to use alternative non-MP partitions of the Hamiltonian to yield variants of BW2 that are size-consistent as well as regular. We show that there is a vast space of such BW2 theories and also show that it is possible to define a repartitioned BW2 theory from the ground state density alone, which regenerates the exact correlation energy. We also provide a general recipe for deriving regular, size-consistent, and size-extensive partitions from physically meaningful components, and we apply the result to small model systems. The scope of these results appears to further set the stage for a revival of BW2 in quantum chemistry.

Inferring interphase chromosomal structure from multiplexed fluorescence <i>in situ</i> hybridization data: A unified picture from human and mouse cells

We analyze multiplexed fluorescence in situ hybridization (m-FISH) data for human and mouse cell lines. The m-FISH technique uses fluorescently-labeled single-stranded probes which hybridize to specific chromosomal regions, thereby allowing the measurement of the spatial positions of up to ∼100 tagged sites for several thousands of interphase chromosomes. Our analysis focuses on a wide range of different cell lines and two distinct organisms and provides a unified picture of chromatin structure for scales ranging from 5 kb (kilobases) up to 2 Mb (megabases), thus covering a genomic region of almost three orders of magnitude. Confirming recent analysis [Remini et al., Phys. Rev. E 109, 024408 (2024)], we show that there are two characteristic arrangements of chromatin referred to as phase α (crumpled globule) and phase β (looped domain) and discuss the physical properties of these phases. We show that a simple heterogeneous random walk model captures the main behavior observed in experiments and brings considerable insights into chromosomal structure.

Comment on “Long-range asymptotics of exchange energy in the hydrogen molecule” [J. Chem. Phys. 152, 174308 (2020)]

Theoretical design of nanocatalysts based on (Fe2O3)<i>n</i> clusters for hydrogen production from ammonia

The catalytic activities of high-spin small Fe(III) oxides have been investigated for efficient hydrogen production through ammonia decomposition, using the artificial force induced reaction method within the framework of density functional theory with the B3LYP hybrid exchange–correlation functional. Our results reveal that the adsorption free energy of NH3 on (Fe2O3)n (n = 1–4) decreases with increasing cluster size up to n = 3, followed by a slight increase at n = 4. The strongest NH3 adsorption energy, 28.55 kcal/mol, was found for Fe2O3, where NH3 interacts with a two-coordinated Fe site, forming an Fe–N bond with a length of 2.11 Å. A comparative analysis of NH3 dehydrogenation and H2 formation on various Fe(III) oxide sizes identifies the rate-determining steps for each reaction. We found that the rate-determining step for the full NH3 dehydrogenation on (Fe2O3)n (n = 1–4) is size-dependent, with the NH* → N* + H* reaction acting as the limiting step for n = 1–3. In addition, our findings indicate that H2 formation is favored following the partial decomposition of NH3 on Fe(III) oxides.

Influence of counterion substitution on the properties of imidazolium-based ionic liquid clusters

Due to their unique physiochemical properties that may be tailored for specific purposes, ionic liquids (ILs) have been investigated for various applications, including chemical separations, catalysis, energy storage, and space propulsion. The different cations and anions comprising ILs may be selected to optimize a range of desired properties, such as thermal stability, ionic conductivity, and volatility, leading to the designation of certain ILs as designer “green” solvents. The effect of counterions on the properties of ILs is of both fundamental scientific interest and technological importance. Herein, we report a systematic experimental and theoretical investigation of the size, charge, stability toward dissociation, and geometric/electronic structure of 1-ethyl-3-methyl imidazolium (EMIM)-based IL clusters containing two different atomic counterions (i.e., bromide [Br−] and iodide [I−]). This work extends our studies of EMIM+ cations with atomic chloride (Cl−) and molecular tetrafluoroborate (BF4−) anions reported previously by Baxter et al. [Chem. Mater. 34, 2612 (2022)] and Zhang et al. [J. Phys. Chem. Lett. 11, 6844 (2020)], respectively. Distributions of anionic IL clusters were generated in the gas phase using electrospray ionization and characterized by high mass resolution mass spectrometry, energy-resolved collision-induced dissociation, and negative ion photoelectron spectroscopy experiments. The experimental results reveal anion-dependent trends in the size distribution, relative abundance, ionic charge state, stability toward dissociation, and electron binding energies of the IL clusters. Complementary global optimization theory provides molecular-level insights into the bonding and electronic structure of a selected subset of clusters, including their low energy structures and electrostatic potential maps, and how these fundamental characteristics are influenced by anion substitution. Collectively, our findings demonstrate how the fundamental properties of ILs, which determine their suitability for many applications, may be tuned by substituting counterions. These observations are critical in the sub-nanometer cluster size regime where phenomena do not scale predictably to the bulk phase, and each atom counts toward determining behavior.

Anisotropic activations controlling doublet–quartet spin conversion of linked chromophore-radical molecular qubits in fluid

Light-energy conversion processes causing alternations in spin multiplicity are attracting attention, but the development of quantum sensing technology applicable to fluid environment such as inside cells has been unexploited. How to achieve efficient energy conversion with controlling spin quantum coherence in a noisy condensed system is challenging. In this study, we investigate the effect of molecular motion on electron spin polarization to control quantum information of three-spin qubits in a fluid environment by using steric effects of organic molecules at room temperature. Using time-resolved electron paramagnetic resonance to observe light-induced generation and transfer of quantum entanglement, we directly observed a photoexcited quartet state generated in a radical-chromophore coupled system and clarified details of the electron spin polarization mechanism including a decoherence effect by activation of anisotropic molecular motion by the steric effects.

Mechanism of polymer molecular weight-dependent suppression and promotion of liquid–liquid phase separation of a protein solution by the addition of polymer

Polyethylene glycol (PEG) is a widely used precipitant to concentrate proteins. The effect of PEG is generally understood to be an entropic attraction between proteins due to the depletion effect of PEG around proteins. However, measurements by Bloustine et al. [Phys. Rev. Lett. 96, 087803 (2006)] of the liquid–liquid phase separation (LLPS) temperature have shown that a lysozyme solution is stabilized and destabilized by the addition of low and high molecular-weight PEG, respectively. They also presented a theoretical model of the LLPS temperature as a virial expansion of the free energy and concluded that, in addition to the depletion effect, the attractive interaction between protein and PEG is necessary to explain the experiments. In the present study, theoretical calculations based on liquid-state density functional theory utilizing coarse-grained models are conducted to demonstrate that the protein–PEG effective attraction is responsible for the suppression and promotion of LLPS upon the addition of low- and high-weight PEG, respectively. In contrast, if the interactions between the protein and the PEG are solely due to the excluded volume effect, PEG of any molecular weight destabilizes the solution. These results suggest the necessity to reconsider the conventional understanding of the effects of polymer addition, which have been historically attributed to solely the depletion force.

Effect of active layer thickness on device performance of InSnZnO thin-film transistors grown by atomic layer deposition

Amorphous oxide semiconductors have garnered significant attention in recent years for their potential in flat-panel displays and back-end-of-line-compatible monolithic 3D (M3D) integration applications. This study explores amorphous InSnZnO thin films deposited via plasma-enhanced atomic layer deposition (PEALD) and the development of high-performance PEALD ITZO thin-film transistors (TFTs) with different active layer thicknesses, fabricated under a low thermal budget of 200 °C. By optimizing the deposition process of binary oxides InOx, SnOx, and ZnOx, a shared temperature window of 170–180 °C was identified for ITZO thin-film deposition. The deposited ITZO films, irrespective of thickness, exhibit an amorphous phase. Moreover, a reduction in ITZO film thickness from 24 to 4.8 nm leads to an increase in the optical bandgap from 3.35 to 3.65 eV. The channel thickness significantly impacts the threshold voltage and carrier density of ITZO TFTs. Optimized ITZO TFTs with a 16 nm channel thickness demonstrate excellent electrical performance, including a threshold voltage of −0.58 V, a field-effect mobility of 29 cm2/V s, an on/off ratio exceeding 108, and a subthreshold swing of 74 mV/dec. Furthermore, the optimized ITZO TFT exhibits excellent stability under positive bias stress at 2 MV/cm, with a threshold voltage shift of 0.15 V after 3600 s. Consequently, ALD-based ITZO emerges as a promising channel material for future applications in transparent electronics and flat-panel displays.

Unveiling the face-dependent ice growth kinetics: Insights from molecular dynamics on the basal and prism surfaces

Ice nucleation and growth are critical in many fields, including atmospheric science, cryobiology, and aviation. However, understanding the detailed mechanisms of ice crystal growth remains challenging. In this work, crystallization at the ice/quasi-liquid layer (QLL) interface of the basal and primary prism (prism1) surfaces of hexagonal ice (Ih) was investigated using molecular dynamics simulations across a wide range of temperatures for the TIP4P/Ice model, with comparisons to the mW coarse-grained model. Together with elucidating the temperature-dependent mechanisms of crystallization, face-specific growth rates were systematically estimated. While the prism surface generally exhibits faster growth rates than the basal surface, a temperature-dependent crossover in growth rates between the basal and prism surfaces is observed in TIP4P/Ice simulations, which correlates with crossovers in QLL thickness and properties and with the well-known column to platelets transition in ice-crystal habits at low vapor pressure. This observation helps decode the complex dependence between crystal morphology and temperature in ice crystals.

Probing specific ion effects at air-aqueous dibutyl phosphate interfaces using vibrational sum frequency generation spectroscopy

Molecular properties at air–liquid and liquid–liquid interface hold the key to many processes involving molecular transport across phase boundaries from aerosol formation to carbon cycling and material separation using solvent extraction techniques. Using dibutyl phosphate (DBP) as a representative for partially aqueous soluble surfactants, the specific ion effect (SIE) of the Hofmeister series cations Cs+, Na+, Li+, and Mg2+ on the partition and interaction between surfactant molecules and water molecules in the air–aqueous interface are investigated using vibrational sum frequency generation spectroscopy and surface tension measurements. In the presence of 1 mM and 1M bulk aqueous phase ionic strength salt concentrations, fundamental qualitative relationships are observed for the salting out of DBP relative to bulk aqueous phase nitrate salt concentrations and the specific cations species. At 1 mM ionic strength, the interfacial charge and hence the interfacial potential modulates the electrostatic interactions; in particular, the counter cations partially screen the negatively charged interface induced by the DBP in a direct Hofmeister order. At 1M ionic strength, the electric field at the interface or interfacial potential is effectively neutralized, and the counter cations promote the partitioning of DBP to the interface depending on their specific interaction with the DBP head group and metal ion hydration properties. The present results lay a foundation to study SIEs of heavier metals on hydrophobic-aqueous DBP interfaces.

Excited state dynamics of a Bodipy derivative with a twisted molecular structure: Combined experimental and theoretical studies

The photophysical properties of a boron dipyrromethene (Bodipy, BDP) derivative (BDP-SA) in which one F atom at the BDP core was replaced by an O atom and condensed with salicylaldehyde were investigated. This compound has a twisted molecular structure and unusually low fluorescence quantum yield (1% in toluene). No intersystem crossing was observed with a nanosecond transient absorption study. The triplet state lifetime of BDP-SA was determined to be 115 μs by photosensitizing. Femtosecond transient absorption shows a structure relaxation of ∼1.5 ps for the S1 excited state. Theoretical studies show conical intersections, which are responsible for the efficient non-radiative decay of the S1 state, which has extremely weak fluorescence.

Multiphoton dissociation dynamics of molecular oxygen O2 via two-photon resonant Rydberg states in the UV region

The photodissociation and photoionization of O2 and the subsequent photodissociation of O2+ in the wavelength region of 200 to 240 nm are reported using resonance enhanced multiphoton ionization (REMPI) and velocity map imaging detection. A series of two-photon allowed Rydberg states with principle quantum number n = 3–11 converging to the ground electronic state of O2+X2Πg are used as doorway states to reach the region of superexcited states of O2 in the three-photon energy range of 15.8–18.6 eV. A detailed analysis of the kinetic energy release and anisotropy parameters of photofragments extracted from velocity map images reveals competition between neutral dissociation and autoionization and leads to the identification of different O+ formation channels. Moreover, the measurement of anisotropy parameters for each channel gives additional information on the symmetry of electronic states involved in the absorption process. Formation followed by the dissociation of vibrationally excited O2+ is the strongest channel over the full wavelength range studied. Ground and vibrationally excited O2+(X2Πg, a4Πu, A2Πu) are formed and dissociated to ionic products via one and two-photon processes. Neutral dissociation to form electronically excited atoms is important at the longer wavelengths studied and becomes noticeably less important at shorter wavelengths. These results agree with and expand on a previous study from our lab of O+ formation at a single (2 + 1) REMPI wavelength, and the results obtained in this study are found to complement our study of the electronically analogous counterpart S2, where most of the S+ ions arise from electronically excited S* atoms. The results of this study will also be of use in the pixel-to-velocity calibration of any velocity map imaging apparatus in the wide ultraviolet wavelength regions. Because O2 is a common reactant or product in many molecular dynamics studies, knowledge of its ionization/dissociation pathways at commonly used wavelengths should also be useful in avoiding signal overlap problems.

ML-MCTDH-Aid: An auxiliary package for multilayer multiconfiguration time-dependent Hartree calculations

The multilayer-multiconfiguration time-dependent Hartree (ML-MCTDH) method has garnered significant attention in the realm of theoretical chemistry owing to its powerful ability to perform numerically exact descriptions of multi-dimensional quantum dynamics and exhibit the remarkable performance in simulating the nonadiabatic dynamics of complex systems. Despite the availability of computational packages within the ML-MCTDH framework, executing these calculations seamlessly is not a straightforward task. Typically, substantial efforts are necessitated to configure the correct inputs for ML-MCTDH calculations, which require to correctly define several non-trivial parameters, to reasonably setup the optimal tree expansion of wavefunctions, and to properly select basis function numbers. To address these challenges, we have developed an auxiliary package named ML-MCTDH-Aid, which facilitates the setup of ML-MCTDH calculations using the Heidelberg MCTDH package in a user-friendly manner. This package is primarily tailored to handle the high-dimensional nonadiabatic dynamics governed by the Hamiltonian composed of several electronic states, several vibrational modes and their linear vibronic coupling terms. It automatically generates multiple essential input files, and all the calculations can be performed in an all-in-one black-box easy-to-use manner. To show the utility of the ML-MCTDH-Aid package, we provide a step-by-step tutorial that demonstrates running ML-MCTDH studies on three models. These examples illuminate how the utilization of the ML-MCTDH-Aid package significantly enhances the efficiency and effectiveness of ML-MCTDH calculations. This substantially boosts the accessibility of ML-MCTDH calculations in tackling the high-dimensional quantum dynamics of complex systems.

Spin–lattice relaxation mechanism of magnetic field effects on singlet fission in amorphous molecular semiconductors

Singlet fission (SF) in molecular semiconductors is a photophysical process of spontaneous splitting of the excited singlet state into a pair of triplet excitons (TT-pair). This process is usually strongly influenced by spin-selective back geminate TT-annihilation (TTA). Spin selectivity manifests itself in magnetic field effects (MFEs) on both TTA and SF kinetics, the study of which allows us to reveal some specific features of this kinetics. In our work, we analyze the mechanism of MFE generation in TTA and SF processes in amorphous molecular semiconductors. In this mechanism, the MFEs are assumed to be determined by magnetic field dependent spin–lattice relaxation (SLR) in TT-pairs, generated by the zero-field splitting interaction (in T-excitons), fluctuating due to T-exciton hopping over arbitrarily oriented molecules in amorphous semiconductors. The SLR-transitions are described with a semiempirical model, which makes it possible to obtain the SF-kinetic functions in analytical form. The mechanism of SLR-assisted MFEs is found to be very efficient in TTA and SF processes. The obtained results are analyzed in detail and applied to interpret experimentally observed SF-kinetic dependences in various magnetic fields. In particular, it is shown that the proposed model of SLR-generated MFEs enables one to describe the effect of crossing of SF-kinetic functions, corresponding to different magnetic fields.

Probing electronic and vibrational structures of TaC<i>n</i>−/0 (<i>n</i> = 2–4) using high-resolution photoelectron spectroscopy and theoretical calculations

We report the high-resolution photoelectron spectroscopy of transition metal carbide cluster anions TaCn− (n = 2–4) using a cryogenic ion trap combined with the slow electron velocity imaging (cryo-SEVI) technique. From the vibrationally resolved photoelectron spectra and associated ab initio calculations, the electron affinities of TaCn (n = 2–4) were determined with high precision: 1.818(2), 2.202(5), and 2.431(2) eV, respectively. The electronic and vibrational structures observed in the photoelectron spectra were interpreted using density-functional theory and coupled-cluster singles and doubles with perturbative triples calculations. Both the neutral TaCn clusters and their anions exhibit planar C2v structures, where the Ta atom bridges each C atom. Furthermore, we observed the spin–orbit splitting in the ground state of TaC2 (X̃4B1), with a measured splitting of 256(25) cm−1. This splitting is well explained by the calculated E1/2(±3/2)−E1/2(±1/2) splitting of 216 cm−1, obtained using the MRCI+SOC method.

Gearing of nitrate ions in ammonium nitrate

Reorienting polyatomic ions such as NH4+ and NO3− exhibit weak magnetic fields because the ions at the extremities trace out current loops; if the periodic reorientations become long-range ordered (i.e., gearing of neighboring NO3−), then the magnetic susceptibility should exhibit a unique signature along the different crystallographic axes. For the case of ammonium nitrate (NH4NO3), we report the presence of two successive sharp steps in the molar magnetic susceptibility along the a- and b-axes upon crossing its order–disorder phase transition (from phase IV to phase II). We suggest that the first step pertains to the NO3− planes shifting away from facing only along the b-axis and onto the a-axis by 45°. The second step is attributed to the disordering (ungearing) of the NH4+ and NO3−. In contrast, only one step was observed in the magnetic susceptibility along the c-axis, and its large magnitude suggests that the NO3− remain weakly correlated even in phase I at 400 K. We also find evidence that the NH4+ become magnetically ordered (geared) along the c-axis only until phase V. The approach employed in this work can be extended to experimentally study the lattice dynamics of other solids possessing planar ions such as amphidynamic crystals.