Kuo's Group

The first wetting layer on the GaN (0001) surface has been investigated at the level of density functional theory. Many water adsorption models have been analyzed and it is observed that the number of water molecules that can be dissociated is limited to 0.375 ML of adsorption sites; further water dissociation will cost energy penalty. The coverage of hydroxyl groups on surface could be up to 0.75 ML instead. It is also observed that the additional charge on the surface will totally transfer to water adsorbates when the water dissociation number is 0.375 ML. Meanwhile, the surface states will disappear when all the adsorption sites are occupied by dissociated or intact water. All of these phenomena can be attributed to the electron counting rule of III–V semiconductor growth theory. We suggest that the electron counting rule could be generally applied to the water adsorption on polar III–V and II–VI semiconductor surfaces.

A number of isomer structures can be formed in hydrogen-bonded clusters, reflecting the essential variety of structural motifs of hydrogen bond networks. Control of isomer distribution of a cluster is important not only in practical use for isomer-specific spectroscopy but also in understanding of isomerization processes of hydrogen bond networks. Protonated methanol clusters have relatively simple networks and they are model systems suitable to investigate isomer distribution changes. In this paper, isomer distribution of H+(CH3OH)7 is studied by size-selective infrared spectroscopy in the OH and CH stretching vibrational region and density functional theory calculations. While the clusters produced by a supersonic jet expansion combined with electron ionization were predominantly isomers having open hydrogen bond networks such as a linear chain, the Ar or Ne attachment (so-called rare gas tagging) entirely switches the isomer structures to compactly folded ones, which are composed only of closed multiple rings. The origin of the isomer switching is discussed in terms of thermal effects and specific isomer preference.

With first-principle DFT calculations{,} the catalytic activity of heteroatom-doped carbon nanostructures in oxygen reduction reaction is investigated by exploring the active site of B-doped{,} N-doped and (B{,} N)-codoped and analyzing the kinetic pathways of oxygen reduction with the participation of protons. It is found that the heteroatom-doped graphene can become the effective catalysis materials for ORR with four-electron pathway. Especially{,} the formation of epoxide groups may be important for the four-electron processes on B-doped and (B{,} N)-codoped graphene. By the analysis of charge redistribution{,} the formation of active catalytic sites is attributed to the localized positive charge and electronic dipole induced by the dopant.

In this work{,} we report infrared spectra of large neutral and protonated methanol clusters{,} (MeOH)n and H+(MeOH)n{,} in the CH and OH stretching vibrational region in the size range of n = 10-50. The infrared-ultraviolet double resonance scheme combined with mass spectrometry was employed to achieve moderate size selection of the neutral clusters with the addition of a phenol molecule as a chromophore. Infrared dissociation spectroscopy was performed on the protonated methanol clusters by using a tandem quadrupole mass spectrometer to enable the precise size selection of the clusters. While the neutral clusters showed essentially the same spectra in all the observed size range{,} the protonated clusters showed remarkable narrowing of the H-bonded OH stretch band with increasing n. In n [greater-than-or-equal] [similar]30{,} the spectra of the neutral and protonated clusters become almost identical. These spectral features demonstrate that hydrogen bond networks of methanol prefer simple cyclic structures (or {"}bicyclic{"} structures in protonated methanol) and branching of the hydrogen bond networks (side-chain formation) is almost negligible. Implications of the spectra of the clusters are also discussed by comparison with spectra of bulk phases.

By the incorporation of C into (BN)12 fullerene, our theoretical investigation shows that the hydrogenation reaction on carbon doped \{B11N12C\} cluster is both thermodynamically favored and kinetically feasible under ambient conditions. Without using the metal catalyst, the C atom can work as an activation center to dissociate \{H2\} molecule and provide the free H atom for further hydrogenation on the \{B11N12C\} fullerene, which saves the materials cost in practical applications for hydrogen storage. Moreover, the material curvature also plays an important role in reducing the activation barrier for the hydrogen dissociation on the \{BN\} fullerenes.

With first-principles DFT calculations, the interaction between Li and carbon in graphene-based nanostructures is investigated as Li is adsorbed on graphene. It is found that the Li/C ratio of less than 1/6 for the single-layer graphene is favorable energetically, which can explain what has been observed in Raman spectrum reported recently. In addition, it is also found that the pristine graphene cannot enhance the diffusion energetics of Li ion. However, the presence of vacancy defects can increase the ratio of Li/C largely. With double-vacancy and higher-order defects, Li ion can diffuse freely in the direction perpendicular to the graphene sheets and hence boost the diffusion energetics to some extent.

Due to the severe self-interaction errors associated with some density functional approximations{,} conventional density functionals often fail to dissociate the hemibonded structure of the water dimer radical cation (H2O)2+ into the correct fragments: H2O and H2O+. Consequently{,} the binding energy of the hemibonded structure (H2O)2+ is not well-defined. For a comprehensive comparison of different functionals for this system{,} we propose three criteria: (i) the binding energies{,} (ii) the relative energies between the conformers of the water dimer radical cation{,} and (iii) the dissociation curves predicted by different functionals. The long-range corrected (LC) double-hybrid functional{,} [small omega]B97X-2(LP) [J.-D. Chai and M. Head-Gordon{,} J. Chem. Phys.{,} 2009{,} 131{,} 174105]{,} is shown to perform reasonably well based on these three criteria. Reasons that LC hybrid functionals generally work better than conventional density functionals for hemibonded systems are also explained in this work.

To assess the accuracy of density functional theory (DFT) methods in describing hydrogen bonding in condensed phases{,} we benchmarked their performance in describing phase transitions among different phases of ice. We performed DFT calculations of ice for phases Ih{,} II{,} III{,} VI and VII using BLYP{,} PW91{,} PBE{,} PBE-D{,} PBEsol{,} B3LYP{,} PBE0{,} and PBE0-D{,} and compared the calculated phase transition pressures between Ih-II{,} Ih-III{,} II-VI{,} and VI-VII with the 0 K experimental values of Whalley [J. Chem. Phys.{,} 1984{,} 81{,} 4087]. From the geometry optimization of many different candidates{,} we found that the most stable proton orientation as well as the phase transition pressure does not show much functional dependence for the generalized gradient approximation and hybrid functionals. Although all these methods overestimated the phase transition pressure{,} the addition of van der Waals (vdW) correction using PBE-D and PBE0-D reduced the transition pressure and improved the agreement for Ih-II. On the other hand{,} energy ordering between VI and VII reversed and gave an unphysical negative transition pressure. Binding energy profiles of a few conformations of water dimers were calculated to understand the improvement for certain transitions and failures for others with the vdW correction. We conclude that vdW dispersion forces must be considered to accurately describe the hydrogen bond in many different phases of ice{,} but the simple addition of the R-6 term with a small basis set tends to over stabilize certain geometries giving unphysical ordering in the high density phases.

Boron nitride (BN) domains are easily formed in the basal plane of graphene due to phase separation. With first-principles calculations{,} it is demonstrated theoretically that the band gap of graphene can be opened effectively around K (or K[prime or minute]) points by introducing small BN domains. It is also found that random doping with boron or nitrogen is possible to open a small gap in the Dirac points{,} except for the modulation of the Fermi level. The surface charges which belong to the [small pi] states near Dirac points are found to be redistributed locally. The charge redistribution is attributed to the change of localized potential due to doping effects. The band opening induced by the doped BN domain is found to be due to the breaking of localized symmetry of the potential. Therefore{,} doping graphene with BN domains is an effective method to open a band gap for carbon-based next-generation microelectronic devices.

The effects of adsorbing simple aromatic molecules on the electronic structure of graphene were systematically examined by first-principles calculations. Adsorptions of different aromatic molecules borazine (B3N3H6), triazine (C3N3H3), and benzene (C6H6) on graphene have been investigated, and we found that molecular adsorptions often lead to band gap opening. While the magnitude of band gap depends on the adsorption site, in the case of C3N3H3, the value of the band gap is found to be up to 62.9 meV under local density approximation—which is known to underestimate the gap. A couple of general trends were noted: (1) heterocyclic molecules are more effective than moncyclic ones and (2) the most stable configuration of a given molecule always leads to the largest band gap. We further analyzed the charge redistribution patterns at different adsorption sites and found that they play an important role in controling the on/off switching of the gap—that is, the energy gap is opened if the charge redistributes to between the C–C bond when the molecule is adsorbing on graphene. These trends suggest that the different ionic ability of two atoms in heterocyclic molecules can be used to control the charge redistribution on graphene and thus to tune the gap using different adsorption conditions.

Water decomposition process was investigated by ab initio molecular dynamic simulations using a model of (H2O)2+ clusters. The proton transfer (PT) process from the cationic H-donor water to the H-acceptor water for the formation of (HO[radical dot])[middle dot]H3O+ was predicted as about 90 fs on average calculated at CCSD level of theory. The valence-electron transfer (VET) process through the formation of hemibond interaction between neutral and cationic water{,} (H2O)2+{,} was also identified in several collected trajectories. Both PT and VET processes were found to propagate along two orthogonal reaction coordinates{,} the former was through an intermolecular hydrogen bond and the latter required oxygen-oxygen hemibonding. Significant difference of the theoretical electronic transitions along the VET trajectories was also observed in comparison with the non-VET cases{,} being calculated at SAC-CI level. The strong absorption features of hemibonding (H2O)2+ may introduce an interesting consideration for experimental design to monitor the water decomposition process.

The interaction between graphene and a SiO 2 surface has been analyzed with first-principles DFT calculations by constructing the different configurations based on α-quartz and cristobalite structures. The fact that single-layer graphene can stay stably on a SiO 2 surface is explained based on a general consideration of the configuration structures of the SiO 2 surface. It is found that the oxygen defect in a SiO 2 surface can shift the Fermi level of graphene down which opens up the mechanism of the hole-doping effect of graphene adsorbed on a SiO 2 surface observed in a lot of experiments.

The magic number behavior of ((CH3)3N)n–H+–H2O clusters at n = 3 is investigated by applying infrared spectroscopy to the clusters of n = 1–3. Structures of these clusters are determined in conjunction with density functional theory calculations. Dissociation channels upon infrared excitation are also measured, and their correlation with the cluster structures is examined. It is demonstrated that the magic number cluster has a closed-shell structure, in which the water moiety is surrounded by three (CH3)3N molecules. The ion core (protonated site) of the clusters is found to be (CH3)3NH+ for n = 1–3, but coexistence of an isomer of the H3O+ ion core cannot be ruled out for n = 3. Large rearrangement of the cluster structures of n = 2 and 3 before dissociation, which has been suggested in the mass spectrometric studies, is confirmed on the basis of the structure determination by infrared spectroscopy.

Cluster expansion provides a powerful tool in materials modeling. It has enabled an efficient prediction of the atomic properties of materials with the combination of the modern quantum calculation theory. To construct an accurate cluster expansion model, a few important cluster figures should be identified. This paper proposes a novel figure selection method based on memetic algorithm (MA), which is a synergy of genetic algorithm (GA) and orthogonal matching pursuit (OMP) based memetic operation. The memetic operation is designed to fine-tunes the solutions of GA and accelerate the convergence of the search. The performance of the proposed method is evaluated on two binary alloy datasets. Comparative study to other state-of-the-art figure selection methods demonstrates that the proposed method is capable of obtaining better or competitive prediction accuracy and searching the figure space efficiently.

The hydrogen spillover mechanism on B-doped graphene was explicitly investigated by first-principles calculations. By the incorporation of boron into graphene, our theoretical investigation shows that B doping can substantially enhance the adsorption strength for both H atoms and the metal cluster on the substrate. The firmly bound catalytic metal on B-doped graphene can effectively dissociate H2 molecules into H atoms, and the H atom is more likely to migrate from the bridge site of the H-saturated metal to the supporting graphene sheet. Further investigation on the BC3 sheet gives sufficiently low activation barriers for both H migration and diffusion processes; thus, more H atoms are expected to adsorb on BC3 substrate via H spillover under ambient conditions compared with the undoped graphene case. Our result is in good agreement with recent experimental findings that microporous carbon has an enhanced hydrogen uptake via boron substitution, implying that B doping with spillover is an effective approach in the modification of graphitic surface for hydrogen storage applications.

Using a combination of the bond order–length–strength correlation theory, the spin-polarized tight binding method, the first-principles calculations, and the atomistic photoelectron distillation experiments, we investigated the mechanisms of edge-selective generation and hydrogenated modulation of Dirac-Fermi polarons (DFPs) surrounding the atomic vacancies at a graphite surface and at the edges of graphene nanoribbons (GNR). We found that: (i) the \{DFPs\} with a high-spin density at a zigzag-GNR edge and at an atomic vacancy result from the isolation and polarization of the dangling σ-bond electrons of √3d (d is the C–C bond length) distance along the edge by the locally and densely entrapped bonding electrons; (ii) along an armchair-GNR edge and a reconstructed-zigzag-GNR edge, however, the formation of quasi-triple-bond between the nearest edge atoms of d distance prevents the \{DFPs\} from generation; and (iii) hydrogenation reduces the spin density substantially and turns the asymmetric dumb-bell-like density into the spherical-like pz density. A further C 1s photoelectron spectroscopic purification has confirmed that the generation of the \{DFPs\} is associated with two extra peaks of energy states located at the bottom and the top edge of the C 1s band.

The local structures between nano-TiO2 and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMI+TFS–) and 1-butyl-3-methylimidazolium trifluoromethanesulfonate (BMI+TFS–) were investigated using high-pressure infrared spectroscopy. No significant changes in C–H spectral features of EMI+TFS– were observed in the presence of nano-TiO2 under ambient pressure. As the EMI+TFS–/nano-TiO2 mixture was compressed to 0.3 GPa, the imidazolium C–H absorptions became two sharp bands at 3108 and 3168 cm–1, respectively, and the alkyl C–H stretching absorption exhibits a new band at 3010 cm–1 associated with a weaker band at 3028 cm–1. It appears that pressure stabilizes the isolated conformations due to pressure-enhanced imidazolium C–H–-nano-TiO2 interactions. Our results also reveal that alkyl C–H groups play non-negligible roles at the conditions of high pressures. The results of BMI+TFS–/nano-TiO2 are remarkably different from what is revealed for EMI+TFS–/nano-TiO2. The spectral features and band frequencies of BMI+TFS–/nano-TiO2 are almost identical to those of pure BMI+TFS– under various pressures. This study demonstrates that changes to the alkyl chain length of the cation could be made to control the order and strength of ionic liquid/nano-TiO2 interactions.

The opening of an electrical band gap in graphene is crucial for its application for logic circuits. Recent studies have shown that an energy gap in Bernal-stacked bilayer graphene can be generated by applying an electric displacement field. Molecular doping has also been proposed to open the electrical gap of bilayer graphene by breaking either in-plane symmetry or inversion symmetry; however, no direct observation of an electrical gap has been reported. Here we discover that the organic molecule triazine is able to form a uniform thin coating on the top surface of a bilayer graphene, which efficiently blocks the accessible doping sites and prevents ambient p-doping on the top layer. The charge distribution asymmetry between the top and bottom layers can then be enhanced simply by increasing the p-doping from oxygen/moisture to the bottom layer. The on/off current ratio for a bottom-gated bilayer transistor operated in ambient condition is improved by at least 1 order of magnitude. The estimated electrical band gap is up to ∼111 meV at room temperature. The observed electrical band gap dependence on the hole-carrier density increase agrees well with the recent density-functional theory calculations. This research provides a simple method to obtain a graphene bilayer transistor with a moderate on/off current ratio, which can be stably operated in air without the need to use an additional top gate.

The adsorption mechanism of water on the GaN (0001) polar surface is investigated via both the Density Functional Theory (DFT) method and its derived classical force field. The physisorption binding energy and the adsorption geometry of the water molecule on the clean Ga-terminated surface are analyzed via the first-principle static calculations. The adsorption energy hypersurfaces are then extracted to be used in the fitting of the interaction potentials between water and GaN. Classical molecular dynamics (MD) simulations based on the developed force field are performed for the interfacial system of liquid water and the GaN surface slab. From our computations, the interfacial water exhibits significant oscillatory profiles for the atomic densities and the molecular orientations. Further data analysis suggests a highly confined first layer with the O being locked right upon the surface Ga atoms and the H pointing toward the neighboring O to form the weakened hydrogen bonds. A bilayer configuration with opposite dipole orientations is consequently characterized as the wetting structure on the GaN polar surface and is explained by the anisotropic perturbations from the surface polar sites. Our simulations would be helpful to provide an atomistic picture for the water adsorption configuration on this semiconductor surface and would be useful in the relevant nanofluidic and nanoengineering applications.

Room temperature ferromagnetism in both transition-metals doped and undoped semiconductor thin films and nanostructures challenges our understanding of the magnetism in solids. In this report, we performed the magnetic measurement and Andreev reflection spectroscopy study on undoped Indium–Tin oxide (ITO) thin films and bulk samples. The magnetic measurement results of thin films show that the total magnetization/cm2 is thickness independent. Prominent ferromagnetism signal was also discovered in bulk samples. Spin polarized electron transports were probed on İTO\} thin film/superconductor interface and bulk samples surface/superconductor interface. Based on the magnetic measurement results and spin polarization measurement data, we propose that the ferromagnetism in this material originates from the surface spin polarization and this surface polarization may also explain the room temperature ferromagnetism discovered in other undoped oxide semiconductor thin films and nanostructures.

The structural and dynamic properties of water on the GaN(0001) polar surface are investigated via classical molecular dynamics simulations. The interfacial molecules are observed to have enhanced structural ordering and slowed-down dynamics compared to the liquid bulk; these unique properties are evidenced in the slower reorientational relaxation, smaller diffusion constant, and longer residence lifetime for water located at the surface region up to ∼7 Å from the substrate. Further analysis of the vibrational spectra at low frequencies shows that both the hydrogen bond network bending and the hydrogen bond stretching bands at the interface shift to the blue compared to those in the bulk, due to the strong coupling between the O atom of water and the Ga sites. The distinct spectral features along with the anisotropy of the hydrogen bond distributions of the interfacial water are complex results determined by both the substrate–water and water–water interactions.

The nature of water networks exposed to ionizing radiation is important in various radiation-related chemistry and biology. To understand structural evolution of ionized water networks at the molecular level{,} we report here infrared spectra of water cluster radical cations (H2O)n+ (n = 3 - 11) in the gas phase. Spectral features of free OH stretch modes are quite similar to those of protonated water clusters H+(H2O)n{,} of which the hydrogen-bond network structures have been revealed. In addition{,} we observed an extra band attributed to the stretch of an OH radical in (H2O)n+. These results indicate that nominal (H2O)n+ should be regarded as H+(H2O)n-1(OH) motifs having similar network shapes to those of H+(H2O)n. We also analyzed hydrogen-bonded OH stretch bands and found that hydrogen-bond strength is a key factor to determine the position of the OH radical relative to the protonated site (H3O+/H5O2+). Because an OH radical is a weaker hydrogen bond acceptor than water{,} the first solvation shell of the protonated site is preferentially filled with water. As a result{,} the OH radical is separated from the protonated (charged) site by at least one water molecule in n [greater-than-or-equal] 5 clusters. This result shows the instability of the H3O+-OH ion-radical contact pair in water networks{,} and implies the higher mobility of the OH radical due to its release from the charged site. Observed structural preferences are confirmed both in cold and warm cluster ion sources.