Optoelectronic Properties of Nitrogen-Doped Hexagonal Graphene Quantum Dots: A First-Principles Study.

Graphene quantum dots have been widely studied owing to their unique optical, electrical, and optoelectrical properties for various applications in solar devices. Here, we investigate the optoelectronic properties of hexagonal and nitrogen-doped graphene quantum dots using the first-principles method. We find that doping nitrogen atoms to hexagonal graphene quantum dots results in a significant red shift toward the visible light range as compared to that of the pristine graphene quantum dots, and the doped nitrogen atoms also induce a clear signature of anisotropy of the frontier orbitals induced by the electron correlation between the doped nitrogen atoms and their adjacent carbon atoms. Moreover, time-dependent density functional theory calculations with the M06-2X functional and 6-311++G(d,p) basis set reproduce well the experimental absorption spectra reported recently. These results provide us with a novel approach for more systematic investigations on next-generation solar devices with assembled quantum dots to improve their light selectivity as well as efficiency.

Binding Mechanism and Surface-Enhanced Raman Scattering of the Antimicrobial Sulfathiazole on Gold Nanoparticles.

A combined study using the surface-enhanced Raman scattering (SERS) technique and quantum chemical calculations was carried out to elucidate the adsorption behavior of sulfathiazole, an antibiotic drug, on gold nanoparticles. The tetrahedral Au20 cluster was used as a simple model to mimic a nanostructured gold surface. Computations using density functional theory with the PBE functional were performed in both the gas phase and aqueous medium using a continuum model. The drug is found to bind to the Au metals via the nitrogen of the thiazole ring. The interaction is also partially stabilized by the ring-surface π coupling rather than a sideway adsorption as previously proposed. In an aqueous solution, the drug molecule mainly exists as a deprotonated form, which gives rise to a much greater affinity toward Au nanoparticles as compared to the neutral forms. The drug adsorption further induces a significant alteration on the energy gap of the gold cluster Aun, which could result in an electrical noise. Notable SERS signals below 1600 cm-1, which result from a coupling of several vibrations including the ring breathing, C-C stretching, and N-H bending, could be employed for both qualitative and quantitative detection and assessment of sulfathiazole at trace concentrations.

Theoretical study of the binding mechanism between anticancerous drug mercaptopurine and gold nanoparticles using a cluster model.

CONTEXT: Mercaptopurine is an effective anticancer medicine yet known with serious adverse reactions, thus requiring further attempts to enhance its biological targeting. Small gold clusters Aun (n = 2-10) were used as model reactants to simulate the surface of gold nanoparticles. The computed results show that the drug molecules tend to anchor on the gold clusters at the S atom with the associated binding energies varying from -50 to -34 kcal mol-1 (in vacuum) and from -42 to -28 kcal mol-1 (in aqueous solution). Furthermore, the adsorption of the drug onto the gold surface is considered as a reversible process, and the mechanism of drug releasing was found to be triggerable by internal factors, such as a pH change or the concentrated presence of thiol amino acids in cancerous protein structures. METHOD: Calculations based on density functional theory (DFT) were performed to probe the nature of interactions between the drug and gold nanoparticles. Structural features, thermodynamic parameters, bonding characteristics, and electronic properties of the resulting complexes were investigated at the PBE//cc-pVTZ/cc-pVDZ-PP level.

A new look at the structure of the neutral Au18 cluster: hollow versus filled golden cage.

The geometry of the neutral Au18 gold cluster was probed by a combination of quantum chemical calculations and far-infrared multiple photon dissociation (FIR-MPD) spectroscopy of a Kr messenger complex. Two low-lying isomers are identified to potentially contribute to the experimental IR spectrum, both being derived from a star-like Au17 structure upon capping with one extra Au atom either inside (18_1) or outside (18_5) the star. In particular, the present detection of structure 18_1 by DFT computations where a golden cage encapsulates an endohedral Au atom, is intriguing as a stable core-shell isomer has, to our knowledge, never been found before for such small neutral gold clusters. DFT and local coupled-cluster (DLPNO and PNO-CCSD(T)) computations indicate that both Au18 isomers are close to each other, within ∼3 kcal mol-1, on the energy scale. Although the exact energy ordering is again method-dependent and remains, at present, inconclusive, the most striking spectral signatures of both isomers are related to vibrational modes localized at atoms capping the inner pentaprism sub-structure that result in prominent peaks centered at ∼80 cm-1, close to the most prominent experimental feature found at 78 cm-1. The calculated IR spectra of both core-shell and hollow isomers are very similar to each other and both agree comparably well with the experimental FIR-MPD spectra of the Au18Kr1,2 complexes.

Binding mechanism and SERS spectra of 5-fluorouracil on gold clusters.

The adsorption behaviour of the 5-fluorouracil (5FU) on small gold clusters Au N with N = 6, 8, 20 was evaluated by means of density functional theory using the PBE-D3 functional in combination with a mixed basis set, i.e. cc-pVDZ-PP for gold atoms and cc-pVTZ for non-metal elements. The binding energies between 5FU and gold clusters were determined in the range of 16-24 and 11-19 kcal/mol in gas-phase and aqueous media, respectively. The corresponding Gibbs energies were found to be around -7 to -10 kcal/mol in vacum and sigificantly reduced to -1 to -6 kcal/mol in water solution, indicating that both the association and dissociation processes are likely spontaneous. An analysis on the charge density difference tends to confirm the existence of a charge transfer from the 5FU molecule to Au atoms. Analysis of the surface-enhanced Raman scattering (SERS) spectra of 5FU adsorbed on the Au surfaces shows that the stretching vibrations of N-H and C=O bonds play a major role in the SERS phenomenon. A mechanism for the drug releasing from the gold surfaces is also proposed. The process is triggered by either the low pH in cancerous tumors or the presence of cysteine residues in protein matrices.

Unexpected structures of the Au17 gold cluster: the stars are shining.

The Au17 gold cluster was experimentally produced in the gas phase and characterized by its vibrational spectrum recorded using far-IR multiple photon dissociation (FIR-MPD) of Au17Kr. DFT and coupled-cluster theory PNO-LCCSD(T)-F12 computations reveal that, at odds with most previous reports, Au17 prefers two star-like forms derived from a pentaprism added by two extra Au atoms on both top and bottom surfaces of the pentaprism, along with five other Au atoms each attached on a lateral face. A good agreement between calculated and FIR-MPD spectra indicates a predominant presence of these star-like isomers. Stabilization of a star form arises from strong orbital interactions of an Au12 core with a five-Au-atom string.

Structural characteristics and chemical reactivity of gold-based clusters Aun (n = 16, 17) toward lone pairs.

Gold and gold-based clusters are still receiving special attention owing to their diverse applicability. Density functional theory (DFT) calculations were employed to investigate the structures, the lone-pair affinity of some small Aun clusters (n = 16, 17, and the effects of chromium dopant on their properties. Regarding Au16, the boat-like Cs conformation and the hollow Td form are found to be energetically quasi-degenerate and strongly competing as the ground state. In terms of Au17, a star-like cage is computed to be more stable than the putative global minimum reported in preceding studies. Given Au16Cr isomer, a novel structure for the lowest energy is proposed. Binding energies of these species with CO, NH3, and PH3 ligands are also provided as an in-depth reference to the literature. Present results altogether encourage further theoretical implementations, e.g., CCSD(T) and experimental probes, e.g., the infrared, to validate the findings, thus harnessing the potentiality of the complexes.

The lowest-energy structure of the gold cluster Au10: planar vs. nonplanar?

The onset of the transition from 2D to 3D structures in pure gold clusters remains a matter of continuing debate. In this theoretical study we revisit several planar and non-planar structural motifs of the size Au10 with the aim to clarify this issue. Computations using a long-range corrected exchange-correlation functional LC-BLYP, coupled-cluster theories CCSD(T) and PNO-LCCSD(T)-F12 reveal that, at variance with previous reports on the preference of a planar elongated hexagon, both planar and nonplanar isomers of the neutral Au10 are energetically degenerated up to 300 K. Its 3D equilibrium geometry is a core-shell structure which can be built up from a trigonal prism by capping four extra Au atoms outside. A comparison to the available experimental vibrational spectra allows us to argue that both lowest-lying 2D and 3D isomers of Au10 likely coexist in the molecular beam during measurement of its FIR spectra. This result also suggests that the 2D-3D transition of neutral gold clusters occurs at the size Au10.

The binary boron lithium clusters B12Lin with n = 1-14: in search for hydrogen storage materials.

Molecular structures and properties of the binary clusters containing twelve boron atoms mixed with n lithium atoms, B12Lin with n = 1-14, were investigated using density functional theory with the TPSSh functional and the 6-311+G(d) basis set. Energetic parameters including relative energies, average binding energies and second-order energies of the entire series were predicted using the coupled-cluster theory (U)CCSD(T) in conjunction with the cc-pVTZ basis set. Several lowest-lying isomers were determined for each size B12Lin whose energies differ from each other by <3 kcal mol-1, except for n = 1, 2 and 4 (≤5 kcal mol-1), and particularly n = 8 (∼13 kcal mol-1). Electronic structure and chemical bonding in some specific sizes such as B12Li4, B12Li8 and B12Li14 were analyzed in detail. We established the electron shells of some magic clusters such as the B12Li4 cone for which we proposed a mixed cone-disk electron shell model. Thanks to both the phenomenological shell and Clemenger-Nilsson models, B12Li8 which contains a specific set of shells of 44 valence electrons is a high stability species. The arrangement of Li atoms around a fullerene B12 framework shows that the mixed B12Li8 emerges as the most suitable of this cluster series to adsorb molecular hydrogen. Up to 32 H2 molecules can strongly be attached to the B12Li8 cluster which is thus predicted to be a realistic candidate for hydrogen storage material with gravimetric density reaching up to a theoritical limit of 26 wt%. Attachment of the fifth H2 molecule to each Li atom of B12Li8 results in weaker average bonds but can give rise to a total of 40 H2 molecules, corresponding to 30 wt% of hydrogen.

Adsorption/Desorption Behaviors and SERS Chemical Enhancement of 6-Mercaptopurine on a Nanostructured Gold Surface: The Au20 Cluster Model.

Computational approaches are employed to elucidate the binding mechanism and the SERS phenomenon of 6-mercaptopurine (6MP) adsorbed on the tetrahedral Au20 cluster as a simple model for a nanostructured gold surface. Computations are carried out in both vacuum and aqueous environments using a continuum model. In the gaseous phase and neutral conditions, interaction of 6MP with the gold cluster is mostly dominated by a covalent Au-S bond and partially stabilized by the Au⋅⋅⋅H-N coupling. However, in acidic solution, the nonconventional Au⋅⋅⋅H-S hydrogen-bond becomes the most favorable binding mode. The 6MP affinity for gold clusters decreases in the order of vacuum > neutral solution > acidic medium. During the adsorption, the energy gap of Au20 substantially declines, leading to an increase in its electrical conductivity, which can be converted to an electrical noise. Moreover, such interaction is likely a reversible process and triggered by either the low pH in sick tissues or the presence of cysteine residues in protein matrices. While N-H bending and stretching vibrations play major roles in the SERS phenomenon of 6MP on gold surfaces in neutral solution, the strongest enhancement in acidic environment is mostly due to an Au⋅⋅⋅H-S coupling, rather than an aromatic ring-gold surface π overlap as previously proposed.

Another look at energetically quasi-degenerate structures of the gold cluster Au27 q with q = 1, 0, -1.

Quantum chemical computations were used to reinvestigate the geometries, spectroscopic, and energetic properties of the gold clusters Au27 q in three charge states (q = 1, 0, -1). Density functional theory (DFT) and the domain-based local pair natural orbital modification of the coupled-cluster theory DLPNO-CCSD(T) calculations revealed that, at variance with earlier reports in the literature, while the anion Au27 - tends to exist in a tube-like form, both the lowest-energy Au27 and Au27 + isomers exhibit a pyramidal shape. However, several isomers were found to lie very close in energy, thus rendering a structural transition and their coexistence easy to occur. More specifically, the equilibrium geometry of the neutral Au27 is a core-shell pyramid-like structure with one gold atom located inside. We also identified a novel ground state for the anion Au27 - and located for the first time the global minimum of the cation Au27 + . The vertical detachment energies of the neutral and anionic states were also computed and used to assign the available experimental photoelectron spectra. Although many Au27 isomers were predicted to be energetically quasi-degenerate, the corresponding distinctive vibrational signatures can be used as fingerprints for the identification of cluster geometrical features.

Comment on 'Structural characterization, reactivity and vibrational properties of silver clusters: a new global minimum for Ag16' by P. L. Rodríguez-Kessler, A. R. Rodríguez-Domínguez, D. MacLeod Carey and A. Muñoz-Castro, Phys. Chem. Chem. Phys., 2020, 22, 27255, DOI: D0CP04018E.

A recent paper by Rodríguez-Kessler et al., Phys. Chem. Chem. Phys., 2020, 22, 27255-27262, reported not only results of quantum chemical computations (using the PW91 density functional) on Ag16 clusters as emphasized in the article's title, but also on the Ag15 size. These authors confirmed previous results obtained by McKee and Samokhvalov (J. Phys. Chem. A, 2017, 121, 5018-5028 using the M06 density functional) that the most stable isomer of Ag15 is a C2v structure. We wish to point out that two low symmetry isomers of Ag15 that have a similar energy content are even lower in energy than their reported C2v global minimum. The relative energies between low-lying Ag15 isomers were again found to be method-dependent, and within the expected accuracy of DFT and CCSD(T) methods they could be considered as energetically degenerate, and likely coexist in a molecular beam. The new lower-energy Ag15 isomers appear to fit more consistently within the structural evolution of small silver clusters.

Theoretical Study of the Binding of the Thiol-Containing Cysteine Amino Acid to the Silver Surface Using a Cluster Model.

Computational approaches within the framework of density functional theory (DFT) were employed to elucidate the binding mechanism of the cysteine amino acid on silver nanoparticles using several small silver clusters Agn with n = 2-10 as surface models. The long-range corrected LC-BLYP functional and correlation consistent basis sets cc-pVTZ-PP and cc-pVTZ were used to determine the structural features, energetics, and spectroscopic and electronic properties of the resulting complexes. In vacuum and highly acidic conditions, cysteine molecules prefer to adsorb on silver clusters via their amine group. In aqueous solution, the thiolate head turns out to be the most energetically favorable binding site. The cysteine affinity of silver clusters is greatly altered in different conditions, i.e., acidic solution < vacuum < aqueous solution, and is strongly dependent on the cluster size. As compared to free clusters, the frontier orbital energy gap of the ones capped by cysteine is significantly improved, which corresponds to stronger stability, especially in aqueous solution. The analysis of frontier orbitals also reveals that both forward and backward electron donations exhibit comparable contributions to the enhancement of stabilizing interactions. As for an application, a chemical enhancement mechanism of the surface-enhanced Raman scattering (SERS) procedure of cysteine by silver clusters was also analyzed.

Gold nanoclusters as prospective carriers and detectors of pramipexole.

Pramipexole (PPX) is known in the treatment of Parkinson's disease and restless legs syndrome. We carried out a theoretical investigation on pramipexole-Au cluster interactions for the applications of drug delivery and detection. Three Au N clusters with sizes N = 6, 8 and 20 were used as reactant models to simulate the metallic nanostructured surfaces. Quantum chemical computations were performed in both gas phase and aqueous environments using density functional theory (DFT) with the PBE functional and the cc-pVDZ-PP/cc-pVTZ basis set. The PPX drug is mainly adsorbed on gold clusters via its nitrogen atom of the thiazole ring with binding energies of ca. -22 to -28 kcal mol-1 in vacuum and ca. -18 to -24 kcal mol-1 in aqueous solution. In addition to such Au-N covalent bonding, the metal-drug interactions are further stabilized by electrostatic effects, namely hydrogen-bond NH⋯Au contributions. Surface-enhanced Raman scattering (SERS) of PPX adsorbed on the Au surfaces and its desorption process were also examined. In comparison to Au8, both Au6 and Au20 clusters undergo a shorter recovery time and a larger change of energy gap, being possibly conducive to electrical conversion, thus signaling for detection of the drug. A chemical enhancement mechanism for SERS procedure was again established in view of the formation of nonconventional hydrogen interactions Au⋯H-N. The binding of PPX to a gold cluster is expected to be reversible and triggered by the presence of cysteine residues in protein matrices or lower-shifted alteration of environment pH. These findings would encourage either further theoretical probes to reach more accurate views on the efficiency of pramipexole-Au interactions, or experimental attempts to build appropriate gold nanostructures for practical trials, harnessing their potentiality for applications.

Elucidating the binding mechanism of thione-containing mercaptopurine and thioguanine drugs to small gold clusters.

Density functional theory methods were employed to clarify the adsorption/desorption behaviors of the thione-containing mercaptopurine and thioguanine drugs on the gold surface using both small Au6 and Au8 clusters as model reactants. Structural features, thermodynamic parameters, bonding characteristics, and electronic properties of the resulting complexes were investigated using the Perdew-Burke-Ernzerhof (PBE) and LC-BLYP functionals along with correlation-consistent basis sets, namely cc-pVDZ-PP for gold and cc-pVTZ for non-metals. Computed results show that the drug molecules tend to anchor on the gold cluster at the S atom with binding energies around -34 to -40 kcal/mol (in vacuum) and - 28 to -32 kcal/mol (in aqueous solution). As compared to Au8 , Au6 undergoes a shorter recovery time and a larger change of energy gap that could be converted to an electrical signal for selective detection of the drugs. Furthermore, interactions between the drugs and gold clusters are reversible processes and a drug release mechanism was also proposed. Accordingly, the drugs are able to separate from the gold surface due to either a slight change of pH in tumor cells or the presence of cysteine residues in protein matrices.

A computational study of thiol-containing cysteine amino acid binding to Au6 and Au8 gold clusters.

Density functional theory (DFT) calculations are employed to examine the adsorption behaviors of cysteine on the gold surface using Au6 and Au8 species as model reactants. Computed results show that cysteine molecules prefer to bind with gold clusters via the S-atom of the thiol group in vacuum and thiolate group in water. The gas-phase adsorption energies are around 20.2 kcal/mol for Au6 and 24.4 kcal/mol for Au8. In water environment, such values are slightly reduced for Au6 (19.6 kcal/mol), but increased a little more for Au8 (25.6 kcal/mol). As a result, if a visible light with a frequency of ν ≈ 6 × 1014 Hz (500 nm) is applied, the time for the recovery of Au6 and Au8 from the most stable complexes will be about 0.38 and 9.3 × 103 s, respectively, at 298 K in water. The Au6 is in addition found to benefit from a larger change of energy gap that could be converted to an electrical signal for detection of cysteine.

Structural Evolution and Stability Trend of Small-Sized Gold Clusters Aun (n = 20-30).

Structural evolution and stability pattern of pure neutral gold clusters Aun in the small size range of n = 20-30 are examined using density functional theory (DFT) calculations. The equilibrium geometries are either confirmed or determined, and some new ground state structures are identified. The most stable configurations of Au21-Au23 sizes are formed by adding extra gold atoms to the highly stable pyramidal structure of Au20, while flat-cage shapes are the best candidates for the global minima of both Au24 and Au25. For larger sizes of n = 26-30, pyramidal motifs tend to dominate the lower-lying population rather than tubular conformations as previously reported. The energy gaps, excitation energies, and exciton binding energies are also computed to test out the performance of the computational methods employed. Accordingly, a density functional with long-range exchange effects is highly recommended to quantitatively investigate both the ground and excited states of pure gold clusters.

Comment on "Theoretical Investigations on Geometrical and Electronic Structures of Silver Clusters".

We would like to make a few comments on the results reported in a recent paper in this journal by Tsuneda, J. Comput. Chem, 2019, 40, 206. The structures of some pure silver clusters were not correctly assigned. © 2019 Wiley Periodicals, Inc.

Elucidation of the molecular and electronic structures of some magic silver clusters Agn (n = 8, 18, 20).

Density functional theory (DFT) calculations were carried out to explore the geometric, spectroscopic, and electronic properties of three magic silver clusters Agn (n = 8, 18, and 20) in detail. The computed results show that the global minima of these clusters are compact, near-spherical structures, while other low-lying isomers exhibit oblate or prolate shapes. Vertical ionization energies for the low-lying isomers were also computed and assigned with respect to available experimental values. Although several isomers were predicted to have similar energies, their electronic and vibrational signatures were quite distinctive, meaning that they could be used as fingerprint signals to distinguish between isomers. In addition, the electronic structures of these systems were explored using the phenomenological shell model. Calculations for the coinage metal clusters M20 (M = Cu, Ag, Au) indicated that the structures and properties of the Ag cluster are similar to those of the Cu cluster in that both Cu20 and Ag20 prefer a compact structure whereas Au20 prefers to adopt a tetrahedral form. Graphical abstract Shell Orbitals of Ag8 Cluster.