The enhancement of nonlinear optical properties of azulene-based nanographene by N atoms: a finishing touch.

Nonlinear optical (NLO) materials play an increasingly important role in optoelectronic devices, biomedicine, micro-nano processing, and other fields. The development of organic materials with strong second or (and) third NLO properties and a high stability is still challenging due to the unknown strategies for obtaining enhanced high order NLO properties. In the present work, π-conjugated systems are constructed by doping boron or (and) nitrogen atoms in the azulene moiety of azulene-based nanographenes (formed with an azulene chain with two bridging HCCHs at the two sides of the connecting CC bonds between azulenes, A1A2A3), and the NLO properties are predicted with time-dependent density functional theory based methods and a sum-over-states model. The doping of heteroatoms induces charge redistribution, tunes the frontier molecular orbital energy gap, changes the composition of some frontier molecular orbitals, and affects the NLO properties of those nanographenes. Among the designed nanographenes, the azulene-based nanographene with two nitrogen atoms at the two ends has the largest static first hyperpolarizability (91.30 × 10-30 esu per heavy atom), and the further introduction of two N atoms at the two ends of the central azulene moiety of this nanographene results in a large static second hyperpolarizability while keeping the large static first hyperpolarizability.

The search for a maximum of the D-π-A paradigm for second order nonlinear optical molecular materials.

Push-pull π-conjugated molecules are one of the paradigms of second order nonlinear optical (NLO) materials and have been extensively explored. However, high-performance second order NLO materials with an optimum electron donor (D), π-bridge (π) and acceptor (A) under this paradigm are still the most sought-after. In the present work, D-π-A molecules with optimal D, π and A combination for strong second order NLO properties are proposed based on molecular orbital theories. The optimal D-π-A push-pull molecule achieves an unprecedentedly strong NLO response under the D-π-A paradigm, i.e., the static first hyperpolarizability reaches -453.92 × 10-30 esu per heavy atom using azulene as part of the π-bridge and acceptor to synergistically reinforce the strength of the acceptor. The protocols of D-π-A NLO molecule design through frontier molecular orbital matching of D, π and A with optimal combination of electron donating and accepting strengths shed light on future molecular NLO materials exploration. The simulated two-dimensional second order spectra provide useful information (e.g., sum frequency generation) on the applications of those D-π-A push-pull molecules in nonlinear optics.

Electronic Structure Modulation of Nanographenes for Second Order Nonlinear Optical Molecular Materials.

Nanographenes (NGs) have drawn extensive attention as promising candidates for next-generation optoelectronic and nonlinear optical (NLO) materials, owing to its unique optoelectronic properties and high thermal stability. However, the weak polarity or even non-polarity of NGs (resulting in weak even order NLO properties) and the high chemical reactivity of zigzag edged NGs hinder their further applications in nonlinear optics, thus stabilization (lowering the chemical reactivity) and polarizing the charge distribution in NGs are necessary for such applications of NGs. The fusion of heptagon and pentagon endows the azulene with the character of donor-acceptor, and the B=N unit is isoelectronic to C=C unit. The introduction of polar azulene and BN are idea to polarize and stabilize the electronic structure of NGs for NLO applications. In the present review, a survey on the functionalization and applications of NGs in nonlinear optics is conducted. The engineering of the electronic structure of NGs by topological defects, doping and edge modulation is summarized. Finally, a summary of challenges and perspectives for carbon-based NLO nanomaterials is presented.

Spin engineering of triangulenes and application for nano nonlinear optical materials design.

The recently synthesized triangulenes with non-bonding edge states could have broad potential applications in magnetics, spintronics and electro-optics if they have appropriate electronic structure modulation. In the present work, strategies based on molecular orbital theory through heteroatom doping are proposed to redistribute, reduce or eliminate the spin of triangulenes for novel functional materials design, and the role of B, N, NBN, and BNB in such intended electronic structure manipulation is scrutinized. π-Extended triangulenes with tunable electronic properties could be potential nonlinear optical (NLO) materials with appropriate inhibition of their polyradical nature. The elimination of spin is achieved by B, N, NBN, and BNB doping with the intended geometric arrangement for enhanced polarity. Intended doping of BNB results in an optimal structure with large static first hyperpolarizability (〈ß0〉) as well as strong Hyper-Rayleigh scattering (HRS) ßHRS(-2ω; ω, ω) (ω = 1064.0 nm), TG7-BNB-ba with a large 〈ß0〉 (18.85 × 10-30 esu per heavy atom) and ßHRS (1.15 × 10-28 esu per heavy atom) much larger than that of a synthesized triangular molecule (1.12 × 10-30 esu of 〈ß0〉 per heavy atom and 5.04 × 10-30 esu of ßHRS per heavy atom). The strong second order NLO responses in the near-infrared and visible regions, particularly the strong sum frequency generation, make these B or (and) N doped triangulenes promising candidates for the fabrication of novel carbon-based optoelectronic devices and micro-NLO devices.

Colossal In-Plane and Out-of-Plane Shift Photocurrents in Single-Layer Two-Dimensional α-Antimonide Phosphorus.

For materials lacking inversion symmetries, an interband transition induced by a photon may result in excited electrons (holes) experiencing a spatial shift leading to generation of directional photocurrents. This phenomenon known as bulk photovoltaic effect (BPVE) shift photocurrent (SPC) has recently attracted immense attention owing to its potential in generating photovoltages that are not restricted by Shockley-Queisser limitations imposed by materials' electronic band gaps. The BPVE was recently reformulated in a quantum mechanics viewpoint as the change in the geometrical phase upon photoexcitation and can now be promptly calculated from Bloch wave functions generated by first-principles calculations. The SPC of an electron (hole) is robust against crystal defects and impurities both in the interior and the surface and can be less dissipative and ultrafast. Herein, an emergence of colossal SPC in a pristine two-dimensional (2D) single-layer α-SbP crystal is predicted from first-principles calculations. An external electric field is further applied on the 2D crystal, and a large SPC enhancement is achieved. The locations of the SPC peaks due to both in-plane and out-of-plane responses suggest that α-SbP can generate a large photocurrent both in visible-light and ultraviolet regions. Single-layer 2D α-SbP is thus an excellent material for strong SPC. This finding is thus expected to open a pathway to exploring efficient photovoltaic devices based on monolayer α-SbP and similar materials.

Strong second order nonlinear optical properties of azulene-based porphyrin derivatives.

The high stability, feasible modification, and good π-conjugation of porphyrin derivatives render these porphyrin-based nanomaterials suitable as potential third order nonlinear optical (NLO) materials. Introducing an azulene in pristine porphyrins can significantly improve the second order NLO properties of the system, and this is studied in the present work using density functional theory based methods and the sum-over-states model. The relative orientation of azulene plays a determinant role in the enhancement of the static first hyperpolarizability (〈ß0〉), e.g., the 〈ß0〉 per heavy atom increases from 0.31 × 10-30 esu to 9.78 × 10-30 esu. Further addition of metals (Mg and Zn) in these azulene-fused porphyrin systems leads to an even larger 〈ß0〉 per heavy atom of 41.59 × 10-30 esu, much larger than that of a recently reported porphyrin derivative (26.47 × 10-30 esu). A novel strategy to stabilize the electronic structures as well as maintain good second order NLO responses by introducing appropriate metals into the azulene-fused porphyrins is extendable to other similar systems. Strong sum frequency generation and different frequency generations of those azulene-fused porphyrins in visible and near-infrared regions may inspire experimental exploration and related applications of azulene-based porphyrins particularly in biological nonlinear optics.

Tuning the edge states in X-type carbon based molecules for applications in nonlinear optics.

Novel carbon based "X-type" graphene nanoribbons (GNRs) with azulenes were designed for applications in nonlinear optics in the present work, and the second order nonlinear optical (NLO) properties of those X-type GNRs were predicted using the sum-over-states (SOS) model. The GNRs with edge states are feasibly polarized. The effects of zigzag edges on the NLO properties of GNRs are scrutinized by passivation, and the electronic structures of GNRs are modulated with heteroatoms at the zigzag edges for improved stability and NLO properties. Those nanomaterials were further functionalized with electron-donating and electron-withdrawing groups (NH2/NO2) to enhance the NLO responses, and the connection of those functional groups at the azulene ends play a determinant role in the enhancement of the NLO properties of those X-type nanoribbons, e.g., the static first hyperpolarizability (〈ß0〉) changes from -783.23 × 10-30 esu to -1421.98 × 10-30 esu. The mechanism of such an enhancement has been investigated. Through two-dimensional second order NLO spectra simulations, particularly besides the strong electro-optical Pockels effect and optical rectification responses, strong electronic sum frequency generations and difference frequency generations are observed in those GNRs. The strong second order NLO responses of those GNRs in the visible light region bring about potential applications of these carbon nanomaterials in nonlinear nanophotonic devices and biological nonlinear optics.

Enhanced Shift Currents in Monolayer 2D GeS and SnS by Strain-Induced Band Gap Engineering.

Group IV monochalcogenides exhibit spontaneous polarization and ferroelectricity, which are important in photovoltaic materials. Since strain engineering plays an important role in ferroelectricity, in the present work, the effect of equibiaxial strain on the band structure and shift currents in monolayer two-dimensional (2D) GeS and SnS has systematically been investigated using the first-principles calculations. The conduction bands of those materials are more responsive to strain than the valence bands. Increased equibiaxial compressive strain leads to a drastic reduction in the band gap and finally the occurrence of phase transition from semiconductor to metal at strains of -15 and -14% for GeS and SnS, respectively. On the other hand, tensile equibiaxial strain increases the band gap slightly. Similarly, increased equibiaxial compressive strain leads to a steady almost four times increase in the shift currents at a strain of -12% with direction change occurring at -8% strain. However, at phase transition from semiconductor to metal, the shift currents of the two materials completely vanish. Equibiaxial tensile strain also leads to increased shift currents. For SnS, shift currents do not change direction, just as the case of GeS at low strain; however, at a strain of +8% and beyond, direction reversal of shift currents beyond the band gap in GeS occur.

Two-dimensional two-photon absorptions and third-order nonlinear optical properties of Ih fullerenes and fullerene onions.

The third order static and dynamic nonlinear optical (NLO) responses of Ih symmetry fullerenes (C60, C240, and C540) and fullerene onions (C60@C240 and C60@C240@C540) are predicted using the ZINDO method and the sum-over-states model. The static second hyperpolarizability of Ih symmetry fullerenes increases exponentially with fullerene size [from 10.00 × 10-34 esu in C60 to 3266.74 × 10-34 esu ≈ γ0(C60) × 92.63 in C540]. The external fields of strong third order NLO responses of Ih symmetry fullerenes change from ultra-violet (C60) to the visible region (C540) as the fullerene size increases. The outer layer fullerene in the fullerene onions has dominant contributions to the third order NLO properties of the fullerene onions, and the inter-shell charge-transfer excitations have conspicuous contributions to the third order NLO properties. The two-dimensional two-photon absorption spectra of C60 and C240 show that those fullerenes have strong two-photon absorptions in the visible region with short wavelength and in the ultra-violet region.

Chiral heteronanotubes: arrangement-dominated chiral interface states and conductivities.

Structural analogue between pure carbonic nanostructures and their boron nitride counterparts provides possibilities for the fabrication of BCN hetero-nanomaterials, which have attracted widespread interest and been synthesized with stacked-layer, monolayer and tubular morphologies. In this work, the arrangement-dominated chiral interface states and conductivities of BCN heteronanotubes are investigated in detail by first principles calculations. The π-conjugated states can be driven by the high potential barrier of insulating BN domains to form chiral transport states along the interfaces. The emerging antiparallel and parallel chiral interface states play a dominant role for resonant transport and provide possibilities for the formation of chiral currents. Moreover, the unidirectional chiral currents have advantages to induce a magnetic field which can reach over 0.1 T. In contrast to the parallel-arranged chiral heteronanotubes, the antiparallel-arranged chiral heteronanotubes with the same stoichiometry have narrower band-gaps and stronger chiral conductivities. Such arrangement-dominated chiral transport interface states endow CHNTs with potential application in magneto-electronics.

Clarification of the Molecular Doping Mechanism in Organic Single-Crystalline Semiconductors and their Application in Color-Tunable Light-Emitting Devices.

Organic single-crystalline semiconductors with long-range periodic order have attracted much attention for potential applications in electronic and optoelectronic devices due to their high carrier mobility, highly thermal stability, and low impurity content. Molecular doping has been proposed as a valuable strategy for improving the performance of organic semiconductors and semiconductor-based devices. However, a fundamental understanding of the inherent doping mechanism is still a key challenge impeding its practical application. In this study, solid evidence for the "perfect" substitutional doping mechanism of the stacking mode between the guest and host molecules in organic single-crystalline semiconductors using polarized photoluminescence spectrum measurements and first-principles calculations is provided. The molecular host-guest doping is further exploited for efficient color-tunable and even white organic single-crystal-based light-emitting devices by controlling the doping concentration. The clarification of the molecular doping mechanism in organic single-crystalline semiconductor host-guest system paves the way for their practical application in high-performance electronic and optoelectronic devices.

Double-helix PnLin chains: novel potential nonlinear optical materials.

The structures, circular dichroism (CD) spectra and nonlinear optical (NLO) responses of a series of inorganic double-helix chains, PnLin (n = 6-12), have been investigated using the quantum chemistry method. P-P and P-Li interactions play a major role in stabilizing double-helix chains. The distinctive CD spectra of the double-helix frameworks (namely, a sharp negative CD band at short-wavelength region and a positive CD band at long-wavelength region) become obvious with increasing number of PLi units. The NLO response augments with the length of the double-helix chains, and the contribution of the axial component along the chain direction gradually becomes crucial simultaneously. Synergistic effects, a decrease of crucial electronic transition energies and charge transfer excitation give rise to enhanced NLO responses. In particular, the electronic transitions from the highest occupied molecular orbital to the lowest unoccupied molecular orbital make significant contributions not only to the positive CD bands in the long-wavelength region, but also to the NLO responses of the double-helix PnLin (n = 6-12) chains.

Evolution of phosphotriesterase activities of the metallo-ß-lactamase family: A theoretical study.

Metallo-ß-lactamase (MßL) is a eubacterial zinc metallo-hydrolase superfamily. Despite their well-known lactamase activities, MßL family members also have the ability to catalyze phosphotriester hydrolysis with different phosphotriesterase activities. In the present study, based on crystal structure comparisons of the related MßL members, a series of models was constructed and calculated using the density functional theory (DFT) method to explore the relationship between active-site changes and phosphotriesterase activities. These calculations show that the energetic barriers for phosphotriesterase activity are considerably reduced due to active-site differences, which describes an evolutionary trend for the development of phosphotriesterase activity in the MßL superfamily. The key event is the appearance of a specialized and negatively charged residue bridging both zinc ions, which plays the two important roles of maintaining charge balance and stabilizing the binuclear active-site structure. This pathway is also consistent with the evolutionary relationships determined by phylogenetic tree analysis using complete residue sequences. Our studies provide the first methodology to explore the development of a new enzyme activity within a superfamily, and to shed new light on understanding the catalytic mechanism from an evolutionary perspective.

Two-dimensional second-order nonlinear optical spectra: landscape of second-order nonlinear optics.

The landscape of second-order nonlinear optical (2nd NLO) responses of a system can be depicted as two-dimensional second-order nonlinear optical spectra in a range of external fields, and this is difficult to be realized in experiment for a wide range of external fields. In the present study, an efficient method for application of sum-over-states model to simulate electronic two-dimensional NLO (2DNLO) spectra has been developed, and techniques to analyze NLO response-structure correlation have been proposed. This 2DNLO method has been applied to simulate the 2DNLO spectra of a series of typical electron push-pull chromophores under external fields of up to 5.00 eV. The correlation between the NLO properties and structure has been disclosed, and a further strategy to enhance the NLO properties of push-pull chromophores has been proposed.

Mechanism of mechanically induced optoelectronic and spintronic phase transitions in 1D graphene spirals: insight into the role of interlayer coupling.

Graphene spirals (GSs), an emerging carbonic nano-material with a Riemann surface, demonstrate extraordinary topological electronic signatures: interlayer coupling similar to van der Waals (vdW) heterojunctions and intralayer coupling within the spiral conformation. Based on the state-of-the-art first-principles technique, the electronic properties of the periphery-modified GSs with geometry deformation are explored under axial strain. For all GSs, there emerges a remarkable phase transition from metal to semiconductor, due to the attenuation of interlayer "σ-bonds" reducing the interlayer tunneling probability for carriers. Analogous to graphene, GSs consist of bipartite sublattices with carbonic sp2 hybridization as well. Once the balance of the bipartite sublattices is lost, there will emerge intense edge (corner) states, contributed by the pz orbitals. In contrast to isolated graphene nanoflakes, GSs realize the continuous spin-polarized edge (corner) state coupling with 1D morphology. However, the spin-polarization is blocked by the robust interlayer "σ-bonds" so that the spintronic transition takes place until this interlayer coupling is broken. More intriguingly, an indirect-direct bandgap transition is observed, revealing excellent optical on-off features. Their tunable properties provide great potential for their application in optoelectronics, spintronics and chemical or biological sensors.

The hierarchical construction of cross-junctions of molecular wires with covalent and noncovalent interactions at the liquid/solid interface.

Hierarchical networks, constructed by non-covalent bond stabilized cross-junctions of covalent one-dimensional molecular wires, are synergistically formed at the liquid/solid interface through in situ on-surface condensation of aromatic amines and aldehydes. Our investigation demonstrates the significant impact of the concentration and structure of monomers on the hierarchical construction of these nanoarchitectures at the interface.

Implementation of Outstanding Electronic Transport in Polar Covalent Boron Nitride Atomic Chains: another Extraordinary Odd-Even Behaviour.

A theoretical investigation of the unique electronic transport properties of the junctions composed of boron nitride atomic chains bridging symmetric graphene electrodes with point-contacts is executed through non-equilibrium Green's function technique in combination with density functional theory. Compared with carbon atomic chains, the boron nitride atomic chains have an alternative arrangement of polar covalent B-N bonds and different contacts coupling electrodes, showing some unusual properties in functional atomic electronic devices. Remarkably, they have an extraordinary odd-even behavior of conductivity with the length increase. The rectification character and negative differential resistance of nonlinear current-voltage characteristics can be achieved by manipulating the type of contacts between boron nitride atomic chains bridges and electrodes. The junctions with asymmetric contacts have an intrinsic rectification, caused by stronger coupling in the C-N contact than the C-B contact. On the other hand, for symmetric contact junctions, it is confirmed that the transport properties of the junctions primarily depend on the nature of contacts. The junctions with symmetric C-N contacts have higher conductivity than their C-B contacts counterparts. Furthermore, the negative differential resistances of the junctions with only C-N contacts is very conspicuous and can be achieved at lower bias.

On-surface synthesis of two-dimensional imine polymers with a tunable band gap: a combined STM, DFT and Monte Carlo investigation.

Two-dimensional polymers are of great interest for many potential applications in nanotechnology. The preparation of crystalline 2D polymers with a tunable band gap is critical for their applications in nano-electronics and optoelectronics. In this work, we try to tune the band gap of 2D imine polymers by expanding the conjugation of the backbone of aromatic diamines both laterally and longitudinally. STM characterization reveals that the regularity of the 2D polymers can be affected by the existence of lateral bulky groups. Density functional theory (DFT) simulations discovered a significant narrowing of the band gap of imine 2D polymers upon the expansion of the conjugation of the monomer backbone, which has been confirmed experimentally by UV absorption measurements. Monte Carlo simulations help us to gain further insight into the controlling factors of the formation of regular 2D polymers, which demonstrated that based on the all rigid assumption, the coexistence of different conformations of the imine moiety has a significant effect on the regularity of the imine 2D polymers.

Exploring long-wave infrared transmitting materials with AxBy form: First-principles gene-like studies.

Long-wave infrared (8-12 µm) transmitting materials play critical roles in space science and electronic science. However, the paradox between their mechanical strength and infrared transmitting performance seriously prohibits their applications in harsh external environment. From the experimental view, searching a good window material compatible with both properties is a vast trail-and-error engineering project, which is not readily achieved efficiently. In this work, we propose a very simple and efficient method to explore potential infrared window materials with suitable mechanical property by first-principles gene-like searching. Two hundred and fifty-three potential materials are evaluated to find their bulk modulus (for mechanical performance) and phonon vibrational frequency (for optical performance). Seven new potential candidates are selected, namely TiSe, TiS, MgS, CdF2, HgF2, CdO, and SrO. Especially, the performances of TiS and CdF2 can be comparable to that of the most popular commercial ZnS at high temperature. Finally, we propose possible ranges of infrared transmission for halogen, chalcogen and nitrogen compounds respectively to guide further exploration. The present strategy to explore IR window materials can significantly speed up the new development progress. The same idea can be used for other material rapid searching towards special functions and applications.

Theoretical study of electron tunneling through the spiral molecule junctions along spiral paths.

The electronic transport properties of carbohelicenes and heterohelicenes absorbed between two metal electrodes have been investigated by using the nonequilibrium Green's function in combination with the density function theory. The transport properties of the molecular junctions are mainly dependent on the nature of spiral molecules. The detailed analyses of the transmission spectra, the energy levels as well as the spatial distribution of molecular projected self-consistent Hamiltonian explain how the geometry of molecules affects the intra-molecular electronic coupling. The spiral current in the configurations can be achieved by tuning the outer edge states of spiral-shaped molecules. Furthermore, the symmetric current-voltage characteristics are investigated with the bias changing for all devices as well as an negative differential resistance behavior is observed.