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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Physical bending provides a more stable and controllable method to modify the electronic properties and device applications compared to chemical doping/surface modification in traditional optoelectronic materials. The bending effect on the electronic properties and nonlinear optical (NLO) responses of linear porphyrin oligomer is explored using quantum chemistry methods by designing a series of curved structures with different bending degrees. The highest occupied molecular orbital−lowest unoccupied molecular orbital energy gap (Egap) of the linear porphyrin oligomer decreases after being bended to form the curved structures, and the Egap values and orbital distributions show independent behavior on bending degree. In the electronic absorption spectra, the maximum absorptions mainly come from the S0 → S1/S0 → S2 transitions with the characteristic of local excitation. The NLO response presents a tendency to increase and then decrease with decreasing the bending degree, which is in good accordance with the widely used two‐level model. The curved frameworks as well as the results offer guidelines for novel organic NLO device design and fabrication. [ABSTRACT FROM AUTHOR]

Abstract: The electronic properties and second-order nonlinear optical (NLO) responses of B/N-doped zigzag graphene nanoribbon (ZGNR) have been investigated using quantum chemistry methods. The electron-deficient B atoms prefer to form [...]

From the same piece of finite size graphene (F-graphene) sheet through different directions (zigzag edge or armchair edge), (4, 4) and (8, 0) carbon nanotube clips form. The electronic properties of the intermediate structures in the two rolling processes 44 (zigzag) and 80 (armchair) have been investigated using quantum chemistry method. The magnetism of the F-graphene sheet disappears with the rolling operation in 44, while it is maintained throughout the whole rolling operation in 80. Furthermore, the highest occupied molecular orbital (HOMO) α and HOMO β gradually extend to the whole framework from the zigzag edges with the rolling operation in 44, and they gradually localize to the lower and upper half of the framework in 80. Oxygen passivation along the opening of the intermediate structures effectively improves the nonlinear optical (NLO) response of the intermediate structures in both the zigzag and the armchair processes. Oxygen passivation along the armchair opening in 80 enhances the [β.sub.tot] value, yet does not bring essential changes to the electron transitions contributed to the NLO response. Oxygen passivation along the zigzag opening in 44 is able not only to enhance the [β.sub.tot] value but also to change the transition nature of electron excitations with a major contribution to the NLO response. Key words: rolling graphene to carbon nanotube, electronic properties, magnetic properties, nonlinear optical responses. Resume : Un meme feuillet de graphene de taille finie (graphene F) s'enroule dans differents sens (bordures de type << zigzag >> ou de type << fauteuil >>) pour former un nanotube de carbone soit de structure (4,4) ou de structure (8,0). Nous avons etudie au moyen d'une methode quantique les proprietes electroniques des structures intermediaires au cours des deux processus d'enroulement, soit le processus 44 (zigzag) et le processus 80 (fauteuil). Le magnetisme du feuillet de graphene F disparait lors du processus d'enroulement 44, alors qu'il se maintient pendant tout le processus d'enroulement 80. Par ailleurs, lors du processus d'enroulement 44, les orbitales moleculaires les plus hautes occupees HOMO α et HOMO β s'etendent progressivement a toute la structure a partir des bordures en zigzag, tandis que lors du processus 80, elles se localisent progressivement dans les moities inferieure et superieure de la structure. Le long de la bordure ouverte des structures intermediaires, la passivation par l'oxygene ameliore efficacement la reponse optique non lineaire de celles-ci lors des deux processus d'enroulement (en zigzag et en fauteuil). Lors du processus d'enroulement 80, la passivation par l'oxygene le long de la bordure ouverte de configuration de type fauteuil fait augmenter la valeur [β.sub.tot], sans toutefois occasionner de variations importantes des transitions electroniques qui contribuent a la reponse optique non lineaire. Lors du processus d'enroulement 44, la passivation par l'oxygene le long de la bordure ouverte de configuration zigzag non seulement fait augmenter la valeur [β.sub.tot], mais modifie aussi la nature de la transition des excitations electroniques qui contribuent largement fi la reponse optique non lineaire. [Traduit par la Redaction] Mots-cles : enroulement du graphene en nanotube de carbone, proprietes electroniques, proprietes magnetiques, reponses optiques non lineaires., Introduction Because of its exotic electrical properties and chemical tunability, graphene has been the object of extensive scientific research in both experiments and theory. (1-3) Theoretically, graphene is the starting [...]

The electronic structures and nonlinear optical (NLO) responses of eight triangle group‐VI transition metal dichalcogenides (TMDs) lateral‐core–shell heterostructures quantum dots (QDs) [MaX2MbX2 and MXa2MXb2; M (Ma, Mb) = Mo and W; X (Xa, Xb) = S and Se] have been explored by using quantum chemistry method. Frontier molecular orbital (FMOs) analyses indicate that the triangular frameworks with more active W and/or Se atoms at shell have smaller energy gaps than those with Mo and/or S atoms, and the active sites of the ultra‐small TMD heterostructures mainly locate at the edge and/or the corner of the frameworks. Generally speaking, active atoms at the shell are beneficial to the enhancement of the frequency‐dependent first hyperpolarizabilities compared with the active atoms in the cores. First hyperpolarizability density analyses show that the local contributions of the first hyperpolarizability in x direction is mainly located at the left and right corners, while that in y direction is mainly located at the bottom edge and top corner, confirming the important role that the shell of the TMDs lateral‐core–shell heterostructure QDs play in its NLO response. Further, the electronic transitions contributed to the SHG responses also occur at the shell of these triangular frameworks. [ABSTRACT FROM AUTHOR]

Based on the successful achievement of graphene-C60 single-molecule transistors in experiments, a series of double-layered GNR-C60, X/GNR-C60 (GNR = graphene nanoribbon; X = H, O, S, passivated edge atoms) devices were designed and their transport properties investigated by means of density functional theory (DFT) and nonequilibrium Green's function (NEGF) methods. The overall conductivity followed the sequence: H/GNR-C60 > S/GNR-C60 > O/GNR-C60, exactly opposite to the electronegativity order of O > S > H. All the X/GNR-C60 (X = H, O, S) devices presented a multiple negative differential resistance (NDR) feature within the considered bias range of 0.0–1.4 V. The magnitude, position, and number of NDR peaks were remarkably influenced by the passivated edges, electrode linkages, interactions between the upper and lower GNRs, and applied voltage. C60, as a medium, had a blocking action at low bias for all H/GNR-C60 devices, and the NDR character originated from the crossing of the frontier molecular orbitals (FMOs), while it behaved as a conductor at high bias, and the NDR behavior was still derived from the change in FMO distributions along the scatter region. C60 always acted as a blocking body in both O/GNR-C60 and S/GNR-C60 devices at any applied voltage, and their NDR peaks also stemmed from the crossing of the FMOs. In addition, which channel to be mainly used depended on the synergistic effect of the applied bias and connecting positions of the electrodes. All these fascinating properties suggest the materials potential applications in the field of nano-electronic devices. [ABSTRACT FROM AUTHOR]

Pure carbon-based nanomaterials with good π conjugation and thermal stability, e.g., nanotubes and nanoribbons, have long served as promising conjugated materials for application in second-order nonlinear-optical (NLO) devices but suffer from two inherent structural problems: weak polarity (or even nonpolarity) and high chemical reactivity ascribed to zigzag edge states. In the present work, boron nitride (BN) chains are used to divide carbon nanotubes and nanoribbons, forming a functional block to tune the electronic structure, and thus induce charge redistribution or modify the molecular energy gap for second-order NLO materials or integrated electronic materials. A balance between the electronic kinetic stability and second-order NLO properties is established by structural manipulation. The strong second-order NLO responses in the visible and near-infrared regions make these hybridized carbon-based molecules potential NLO materials for applications in biological nonlinear optics. The application of BN to tune the electronic structure of carbon nanomaterials paves a path for fabricating nanoelectronic and nano-NLO devices. [ABSTRACT FROM AUTHOR]

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 (〈β0〉) as well as strong Hyper–Rayleigh scattering (HRS) βHRS(−2ω; ω, ω) (ω = 1064.0 nm), TG7-BNB-ba with a large 〈β0〉 (18.85 × 10−30 esu per heavy atom) and βHRS (1.15 × 10−28 esu per heavy atom) much larger than that of a synthesized triangular molecule (1.12 × 10−30 esu of 〈β0〉 per heavy atom and 5.04 × 10−30 esu of βHRS 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. [ABSTRACT FROM AUTHOR]

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 (〈β0〉), e.g., the 〈β0〉 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 〈β0〉 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. [ABSTRACT FROM AUTHOR]

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 (〈β0〉) 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. [ABSTRACT FROM AUTHOR]

The bulk photovoltaic effect shift current (SC), a constant photocurrent generated by irradiating polar homogeneous non‐centrosymmetric materials has lately emerged as a possible option for next‐generation photovoltaic systems that may potentially overcome the Shockley–Queisser limits. The current work predicts gigantic and anisotropic SC with multiple peaks across a wide range of frequencies in pristine 2D monolayer α‐AsP and α‐NP based on first‐principle calculations. The dominant SC tensors components hosted by 2D monolayer α‐AsP and α‐NP are the in‐plane, and reaches peaks of −154 µАV−2, and 87 µАV−2, respectively, in the UV region. The SC magnitudes of the α‐AsP are not significantly affected by the spin‐orbital coupling, while in α‐NP, no such effect occurs. This study recommends new photovoltaic materials with strong SC peaks spread across the electromagnetic spectrum, largely within the UV region. [ABSTRACT FROM AUTHOR]

Optical nonlinearity of two-dimensional (2D) materials with a permanent dipole is a subject of intense research in the fields ranging from all-optical signal processing to laser nanotechnology due to the potential outstanding performance of 2D materials in nonlinear optical processes, and the feasibility for extensive integration into optoelectronic devices. In the present work, the group-VA binary monolayers are predicted to have giant and broadband second harmonic generation (SHG) properties superior to some of the most popular NLO materials by first-principles calculations. The SHG susceptibility of the α-PAs reaches up to 8.0 × 106 pm2 V−1, which is nearly 1–2 orders of magnitude higher than that of honeycomb monolayer MoS2 and h-BN, and twice larger than the record-high SHG susceptibility of a 2D ferroelectric GeSe monolayer. This intriguing peak is formed by intraband two photon transition ascribed to the relatively small bandgap and large transition matrix elements between valence and conduction band edges. A high-performance nonlinear optical device can be developed using those 2D group-VA monolayer materials with puckled structure and in-plane polarizabilities for multifunctional and integrated optoelectronic applications. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Inducing new nonlinear optical (NLO) properties while maintaining the thermal stability of carbon based nanomaterials broadens the application of carbon nanomaterials in opto-electronics. In the present work, polar azulene defects are introduced in Y-type carbon based molecules to induce polarity thus enhancing the second order NLO properties (for example, the static first hyperpolarizability of Y-type carbon based molecules with azulene defects reaches 4.18 × 10−30 esu per heavy atom while that of p-nitroaniline is 2.97 × 10−30 esu per heavy atom). The addition of an electron acceptor at the heptagon end modifies the π electron distribution of Y-type carbon based molecules and significantly enhances the static first hyperpolarizability from 1.73 × 10−30 esu per heavy atom to 8.87 × 10−30 esu per heavy atom, much larger than that of similarly synthesized NLO compounds. The cause of such enhancement by introduction of electron acceptors has been scrutinized. [ABSTRACT FROM AUTHOR]

This study is to improve understanding of nonlinear optics through investigating the microstructures of carbon disulfide (CS2) liquid, an important nonlinear optical material. First principles methods are employed in this work to investigate the structure, vibrational spectra, and the 2nd hyperpolarizabilites of CS2 clusters up to tetramers. Three types of molecular arrangements of clusters with different sizes are identified. The cluster conformations are prone to inter-converting under external disturbance due to the weak interactions among molecules. The correlation between the nonlinear optical properties and the conformation of CS2 clusters are studied in detail in this work. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Curved π bowl compounds represent another class of the completely conjugated materials with quantum dot nature. Non-equivalent hybridisation type from rim to hub carbon atoms in curved π bowl compound triggers anisotropic physical properties. With density functional method (CAM-B3LYP) and response theory calculations, curved π bowl compounds exhibit large radial and axial component ratio for its polarisability and the second hyperpolarisability. More importantly, they possess larger effective mass second hyperpolarisability (γmass) and nondiagonal components (γxxyy) compared to C60. Except the static properties, the dispersion characters of dynamic cubic response of curved π bowl compounds have been analysed in large frequency range. [ABSTRACT FROM AUTHOR]

Ge–Sb–Te superlattice (GST-SL) is a newly emerging electronic material for nonvolatile phase-change memory with ultralow energy cost. However, its switching mechanism is still unclear with intensive debates. In this work, by first-principles calculations and an electron counting model study, we study the possible mechanism of phase change and the accompanying property transition of GST-SL. GST-SL are separated into individual layers by van der Waals gaps. We demonstrate that both the global chemical stoichiometry of the material and the local chemical stoichiometry of individual layer block are required to have an insulating band gap according to an electron counting model analysis. The electrical property can be adjusted by changing the local stoichiometry, such as producing defects around van der Waals gaps. Inspired by a previous experiment, we propose that a stacking-fault motion can spontaneously alter the band gap and results in a metal–insulator transition. This transition may provide a significant change of carrier concentration and indicate an ultralow energy-consumption process with a low energy barrier. The present investigations reveal a picture of electrical transition in GST-SL and may guide us to improve its device performances. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]