Huanyu Zhou

Thermodynamic properties of molecular crystals are the cornerstone of a broad range of applications, spanning pharmaceutics, organic semiconductors, and many more. The reliable modeling of these properties is usually hindered by collective influences of delicate dispersion interactions and insufficient prior knowledge. Providing reasonable accuracy and minimal reliance on experimental data, lattice dynamics (LD) based on the quasi-harmonic approximation (QHA) and density functional theory (DFT) has been widely and successfully adopted for crystal structure prediction (CSP). However, for polymorphic molecular crystals, it remains challenging to accurately establish the relative stability among multiple polymorphs that are distinguished by subtle energy differences. Despite recent intense efforts to achieve calculations that can reproduce observations in well-characterized systems, a comprehensive study elucidating how predictions are governed by errors within the theoretical framework and details of implementation is lacking. This undermines confidence in the transferability and efficiency of QHA LD workflows proposed in the literature. The need for reliable and predictive calculations necessitates systematic investigation and disentangling of the influences from key parameters such as the choice of basis set and Hamiltonian, so the cost and precision of DFT-based QHA LD calculations can be rationally balanced in practical workflows. In this study, paracetamol polymorphs are adopted as the prototype system, as they are well-characterized and display representative chemical interactions, where covalent bonds, hydrogen bonds, and London dispersions are present with sufficient complexity. The intermixed sources of error underlying free-energy predictions are resolved by comparing the structural, vibrational, and thermochemical properties of paracetamol polymorphs with high-quality references. A hierarchy of atomic and plane-wave basis sets are adopted to highlight the significance of basis set completeness, and results with and without the inclusion of Fock exchange identify the particular importance of Hamiltonian for thermochemical properties. Furthermore, an optimal QHA LD workflow is proposed and tested utilizing these insights, which combines results from plane-wave basis set with semilocal functional and local, well-converged def2-TZVP basis set with hybrid functional to efficiently converge the numerical precision for QHA LD. While demonstrated in detail for paracetamol, conclusions of this study are also expected to be of relevance to a wide range of molecular crystals. These findings rigorously set the stage for affordable and reliable QHA LD workflow for molecular crystals, where fortuitous error cancellations are clearly distinguished from converged levels of theory, thereby clarifying the numerical precision attainable before explicit anharmonic and configurational effects are considered for complex organic systems.

In molecular crystals, many solid state phase transitions can be attributed to the entropy of intermolecular vibrations at low frequencies, where the weak noncovalent interactions dominate, and simulations based on density functional theory (DFT) suffer from relatively large errors. In this work, the precision of two critical computational parameters are evaluated, namely the choice of local basis set and (DFT-based) Hamiltonian. Each is documented in detail for the vibrational properties and thermodynamic stability of two polymorphs of paracetamol, which are chosen due to the representative chemical interactions and the good availability of high-quality reference data. Comparisons are made with experimentally measured low-temperature geometries, Raman spectra, and temperature-pressure phase diagrams. These results highlight the substantial influences of basis set, revealing the importance of both the cardinality (i.e., ζ) and the diffuseness; the failure to account for either factor might lead to unexpected results. The features of low-frequency vibrations that most significantly affect the relative stability of molecular crystals are identified and shown to be captured when the triple-ζ def2-TZVP basis set is adopted; on the contrary, widely used double-ζ basis sets are shown to be inadequate. The improvement attributed to the inclusion of Fock exchange is revealed to be marginal, especially with triple-ζ basis sets. By systematically improving levels of precision, the current work disentangles competing sources of errors impeding accurate crystal structure predictions and polymorph energy rankings. This facilitates the predictability and reliability of DFT in thermodynamic modeling of molecular crystals.

The integration of graphene with other 2D materials has been extensively studied over the past decade to realize high-performance devices unattainable with single materials. Graphene-transition metal dichalcogenides (TMDCs) such as MoS₂, WS₂, MoSe₂, and WSe₂ vertical heterostructures have demonstrated promise in numerous electronic and optoelectronic applications due to the wide bandgap range and strong light–matter interaction in TMDCs, and the ability to form electrostatically tunable junctions with graphene. However, conventional methods for TMDCs growth, including chemical vapor deposition (CVD), electrodeposition, and atomic layer deposition (ALD), require high temperatures, which can degrade graphene's electrical and structural properties. Here, we investigate the impact of sulfur annealing on graphene, revealing significant etching and electrical degradation. Density functional theory (DFT) calculations identify the divacancy defect with two sulfur adatoms (DV-2S) and C–S–C bonds as the dominant defect, differing from the previously reported monovacancy with one sulfur adatom (MV-1S). This defect induces p-doping in graphene, consistent with experimental observations. To address these challenges, we introduce a protective strategy utilizing self-assembled monolayers (SAMs) during annealing, enabling the growth of high-quality WS₂ on graphene via electrodeposition. Our findings provide a foundation for integrating TMDCs with graphene while preserving its properties, advancing high-performance electronic and optoelectronic applications.

This work successfully utilises the DFT/ESM-RISM model to elucidate the mechanisms underpinning the novel ‘self-healing’ regrowth behaviour of crystalline paracetamol post breakage.

The electronic and spintronic properties of the monovacancies in freestanding and isotopically compressed graphene are investigated using hybrid exchange density functional perturbation theory. When the effects of electronic self-interaction are taken into account, an integer magnetic moment of 2 μ_B is identified for a Jahn-Teller reconstructed V₁(5-9) monovacancy in freestanding graphene. For graphene with stable ripples induced by a compressive strain of 5%, a bond reconstruction produces a V₁(55-66) structure for the monovacancy, which is localized at the saddle points of the ripple. The sizeable local distortion induced by reconstruction modifies both the geometric and electronic properties of rippled graphene and quenches the magnetic moment of the vacancy due to the sp³ hybridization of the central atom. The nonmagnetic V₁(55-66) structure is found to be stable on rippled structures, with the formation energy ∼2.3 eV lower than that of the metastable distorted V₁(5-9) structures localized at sites other than the saddle points. The electronic ground state of distorted V₁(5-9) corresponds to a wide range of fractional magnetic moments (0.50-1.25 μ_B). The computed relative stabilities and the electronic and magnetic properties of the V₁(5-9) structures are found to be closely related to their local distortions. This analysis of the fundamental properties of defective graphene under compression suggests a number of strategies for generating regular defect patterns with tuneable magnetic and electronic properties and may, therefore, be used as a novel technique to achieve more precise control of graphene electronic structure for various application scenarios such as transistors, strain sensors, and directed chemisorption.