In recent years, organic solar cells (OSCs) have attracted increasing attention, and the power conversion efficiency (PCE) of OSCs has markedly improved. To enhance the photovoltaic properties of OSCs, it is important to develop the donor materials in the light-harvesting layer, which mainly include conjugated polymers and small molecules (SMs). Compared with polymeric materials, small-molecule materials have been widely investigated for their superior characteristics, such as well-defined molecular structures that can provide good batch-to-batch reproducibility. In this work, we synthesized three SM donor materials with theacceptor-donor-acceptor (A-D-A) structure by employing the trialkylthienyl-substituted benzodithiophene (TriBDT-T) unit as the D-core unit, and rhodanine (RN), cyano-rhodanine (RCN), and 1, 3-indanone (IDO) as the A end groups, respectively. The optical properties, molecular energy levels, and thermogravimetic characteristics of the three SMs were studied; moreover, the blend morphologies and photovoltaic properties of the devices by employing the non-fullerene (NF) acceptor, IT-4F, were systematically investigated. The results showed that 1) the three SMs exhibit good thermal stabilities as evinced by thermogravimetric analysis (TGA), and all decomposition temperatures exceeded 410 ℃; 2) They all exhibit strong and broad absorption in the visible light range (300–700 nm), and show similar molar extinction coefficients; 3) the HOMO levels are -5.47 eV, -5.54 eV, and -5.44 eV for RN, RCN, and IDO, respectively, implying the clear influence of the different end groups for the energy levels of the A-D-A-type SMs; the slight differences in the optical and electrochemical properties of the corresponding donor material could be attributed to the different electron-withdrawing ability of the A-type end groups. When studying the photovoltaic properties, interestingly, the RN:IT-4F blend was found to form fibrillar-like aggregates with appropriate size, and the corresponding devices exhibited desirable short circuit current (J_sc) and thus the highest PCE value of 9.25%; however, large-size aggregates were formed in the RCN:IT-4F and IDO:IT-4F blend films, resulting in a much lower J_sc and fill factor (FF), and the PCE of the corresponding devices were only around 6%. In summary, by introducing RN, RCN, and IDO as the A units, we synthesized a class of TriBDT-T based A-D-A type SMs. This study shows that terminal A units exert influence on the absorption spectra, molecular energy level, and morphologies after blending with the acceptor material, and hence, the corresponding devices exhibit significant differences in photovoltaic performance. This work also provides useful information for the molecular design of SM donor materials.

Nonadiabatic processes widely exist in photochemistry and photophysics. The theoretical treatment of nonadiabatic processes is an important challenge in theoretical chemistry. In nonadiabatic dynamics, the well-known "Born-Oppenheimer approximation" breaks down due to the involvement of strong nuclear-electronic coupled motions. Hence, the development of a theoretical framework is required for the proper treatment of nonadiabatic dynamics. The fewest-switches trajectory surface-hopping method developed by Tully is one of the most widely used methods in the treatment of nonadiabatic processes because of its rather simple numerical implementation. In this approach, the nuclear degrees of freedom are propagated on the potential energy surface of an electronic state using the classical equations of motion, while the electronic degrees of freedom are propagated along the same trajectory according to the time-dependent Schr?dinger equation. Nonadiabatic effects are included by allowing sudden hops between different potential energy surfaces. After averaging over many trajectories, a reasonable description of nonadiabatic dynamics is achieved at low computational cost. Particularly, when we combine the trajectory surface-hopping dynamics with on-the-fly molecular dynamics, it is possible to describe the nonadiabatic dynamics of realistic polyatomic molecular systems at a fully atomic level with all degrees of freedom included. After the introduction of hybrid multiscale methods, the simulation of photochemistry in solutions and in biological systems becomes possible. The simulation results provide important information concerning the nonadiabatic dynamics of realistic polyatomic systems, such as excited-state lifetime, major active molecular motions, reaction channels, and branching ratio. This review article summarizes some progresses in this field. After briefly introducing the basic theory of widely used fewest switches surface-hopping dynamics methods, we mainly focus on several numerical details in the implementation of on-the-fly fewest switches surface-hopping dynamics. The seamless combination of surface-hopping dynamics and electronic-structure calculations is emphasized in this review, rather than an exhaustive discussion of rigorous nonadiabatic dynamics theories. Numerical methods to estimate nonadiabatic coupling terms are discussed, which allow us to perform the trajectory surface-hopping calculations when the nonadiabatic coupling vectors are not available in the electronic structure calculations. Several important issues, such as decoherence corrections, diabatic or adiabatic representations, and initial sampling methods are discussed in detail. We also summarize the theoretical treatment of the nonadiabatic dynamics of some interesting molecular systems, which include the photostability of DNA, photo-isomerization of organic systems, photochemistry of transition metal complexes, and photovoltaics. Finally, we discuss the theoretical challenges of this direct dynamics approach and provide an outlook of this field from our personal perspective.

Cu/ZnO/Al₂O₃ is one of the most widely used catalysts in industrial methanol synthesis. However, the reaction mechanism and the nature of the active sites on the catalyst for this reaction are still under debate. Thus, detailed information is needed to understand the catalytic processes occurring on the surface of this catalyst. H₂ is one of the reaction gases in methanol synthesis. Studies of the activation and dissociation behaviors of H₂ on ZnO surfaces are of great importance in understanding the catalytic mechanism of methanol synthesis. In this work, the activation and dissociation processes of H₂ on a ZnO(10${\rm{\bar 1}}$0) single crystal surface were investigated in-situ using ambient-pressure X-ray photoelectron spectroscopy (APXPS) and scanning tunneling microscopy (STM), two powerful surface characterization techniques. In the APXPS experiments, results indicated the formation of hydroxyl (OH) species on the ZnO single crystal surface at room temperature in 0.3 mbar (1 mbar = 100 Pa) H₂ atmosphere. Meanwhile, STM measurements showed that the ZnO surface was reconstructed from a (1×1) to a (2×1) structure upon introduction of H₂. These observations revealed adsorption behaviors of H₂ the same as those of atomic H on a ZnO(10${\rm{\bar 1}}$0) surface as seen in previous studies, which could be evidence of the dissociative adsorption of H₂ on a ZnO surface. However, H₂O adsorption on ZnO surfaces can also result in the formation of OH species, which can be observed using XPS. The STM results show that the exposure of H₂O also leads to the reconstruction from a (1×1) to a (2×1) structure on the ZnO(10${\rm{\bar 1}}$0) surface upon H₂ introduction. Hence, it is necessary to exclude the influence of H₂O in this work, because there may be trace amounts of H₂O in the H₂ gas. Therefore, we performed a comparative study of H₂ and H₂O on ZnO(10${\rm{\bar 1}}$0) single crystal surface. A downward band bending of 0.3 eV was observed on the ZnO surface in 0.3 mbar H₂ atmosphere using APXPS, while negligible band bending was shown in the case of the H₂O atmosphere. Moreover, thermal stability studies revealed that the OH group formed in the H₂ atmosphere desorbed at a higher temperature than the one resulting from H₂O adsorption, meaning that the two OH groups formed on the ZnO surface were different. Results in this work provide evidence of the dissociative adsorption of H₂ on the ZnO(10${\rm{\bar 1}}$0) surface at room temperature and atmospheric pressure. This is in contrast to previous findings, in which no H₂ dissociation on a ZnO(10${\rm{\bar 1}}$0) surface under ultra-high vacuum conditions was observed, indicating that the activation of H₂ on ZnO surfaces is a pressure dependent process.

Heterogeneous catalysts are usually synthesized by the conventional wet-chemistry methods, including wet-impregnation, ion exchange, and deposition-precipitation. With the development of catalyst synthesis, great progress has been made in many industrially important catalytic processes. However, these catalytic materials often have very complex structures along with poor uniformity of active sites. Such heterogeneity of active site structures significantly decreases catalytic performance, especially in terms of selectivity, and hinders atomic-level understanding of structure-activity relationships. Moreover, loss of exposed active components by sintering or leaching under harsh reaction conditions causes considerable catalyst deactivation. It is desirable to develop a facile method to tune catalyst active site structures, as well as their local chemical environments on the atomic level, thereby facilitating reaction mechanisms understanding and rational design of catalysts with high stability.

Atomic layer deposition (ALD), a gas-phase technique for thin film growth, has emerged as an alternative method to synthesize heterogeneous catalysts. Like chemical vapor deposition (CVD), ALD relies on a sequence of molecular-level, self-limiting surface reactions between the vapors of precursor molecules and a substrate. This unique character makes it possible to deposit various catalytic materials uniformly on a high-surface-area support with nearly atomic precision. By tuning the number, sequence, and types of ALD cycles, bottom-up precise construction of catalytic architectures on a support can be achieved.

In this review, we focus on the design and synthesis of supported metal catalysts using ALD. We first review strategies developed to precisely tailor the size, composition, and structures of metal nanoparticles (NPs) using ALD. Catalytic performances of these ALD metal catalysts are also discussed and compared to conventional catalysts. We highlight synthetic strategies for synthesis of metal single-atom catalysts and bottom-up precise synthesis of dimeric metal catalysts. Their impact on catalysis is discussed. We demonstrate that metal oxide ALD on metal NPs can enhance catalytic activity, selectivity, and especially stability. In particular, we show that site-selective blocking of metal NPs with an oxide overcoat improves selectivity and contributes to an understanding of the distinct functionalities of the low-and high-coordination sites in catalytic reactions on the atomic level. Next, we discuss an effective method to construct bifunctional catalysts via precisely-controlled addition of a secondary functionality using ALD. Finally, we summarize the advantages of ALD for the advanced design and synthesis of catalysts and discuss the challenges and opportunities of scaling up ALD catalyst synthesis for practical applications.

As new types of carbon nanomaterials, carbon quantum dots (CQDs) have received widespread attention for their potential applications in optoelectronic device owing to their unique properties such as long hot-electron lifetime, high electron mobility, tunable bandgap, strong stable florescence, solution-processability, stability, non-toxicity, and low cost. Correspondingly, there has been several interesting developments in researches focusing on CQDs. In this review, we will present an update the on the latest research on the synthesis, morphology, structural characteristics, and optoelectronic properties of CQDs. The latter are determined by quantum confinement effect and surface defects. Using bottom-up synthesis methods, CQDs with higher crystallinity and less surface defects could be obtained by accurately designing the precursors and reaction conditions. The structures could be characterized by high-resolution transmission electron microscopy. Secondly, the latest progress on photoelectric devices, including light-emitting diodes (LEDs), solar cells (SCs), and photodetectors (PDs), are summarized in detail. CQDs-based LEDs are divided into photoluminescence (PL) and electroluminescence (EL) LEDs owing to their different excitation modes. Recently, PL LEDs leveled with developed QDs-based LEDs in both luminous efficiency and color rendering index (CRI). With the discovery of their bandgap emission, CQDs overcame carrier injection, which is determined by surface defects and molecule states, and presented excellent potential in EL applications. Moreover, their broad absorption in the ultraviolet-to-visible light range and high electron mobility make CQDs preferable for improving energy conversion efficiency of SCs and responsivity of PDs. Finally, we delineate current challenges on studying CQDs. Its indefinite fluorescence mechanism and structural characterizations limit the development of CQDs. Furthermore, large-scale synthesis methods for CQDs with high quantum yields and crystallinity are not yet established, which hinders their utility in optoelectronic devices. Moreover, CQDs with narrow emission bandwidth (full width at half maximum, FWHM ≤ 35 nm) still do not exist, which restrains their applications in display and laser. Hence, researches on CQDs-based optoelectronic applications are still in the first stages of development. We hope that this review will indicate future directions and encourage critical thinking to elicit new discoveries on CQDs from both fundamental and applied researches. Consequently, the potential of environment-friendly CQDs can be realized in optoelectronics and more areas.

Organic-inorganic perovskite solar cells (PSCs) have become one of the most promising solar cells, as the power conversion efficiency (PCE) has increased from less than 5% in 2009 to certified values of over 22%. In the typical PSC device architecture, hole transport materials that can effectively extract and transmit holes from the active layer to the counter electrode (HTMs) are indispensable. The well-known small molecule 2, 2', 7, 7'-tetrakis-(N, N-di-4-methoxy-phenyl amino)-9, 9'-spirobifluorene (spiro-OMeTAD) is the best choice for optimal perovskite device performance. Nevertheless, there is a consensus that spiro-OMeTAD by itself is not stable enough for long-term use in devices due to the sophisticated oxidation process associated with undesired ion migration/interactions. It has been found that spiro-OMeTAD can significantly contribute to the overall cost of materials required for the PSC manufacturing, thus its market price makes its use in large-scale production costly. Besides, another main drawback of spiro-OMeTAD is its poor reproducibility.

To engineer HTMs that are considerably cheaper and more reproducible than spiro-OMeTAD, shorter reaction schemes with simple purification procedures are required. Furthermore, HTMs must possess a number of other qualities, including excellent charge transporting properties, good energy matching with the perovskite, transparency to solar radiation, a large Stokes shift, good solubility in organic solvents, morphologically stable film formation, and others. To date, hundreds of new organic semiconductor molecules have been synthesized for use as HTMs in perovskite solar cells. Successful examples include azomethine derivatives, branched methoxydiphenylamine-substituted fluorine derivatives, enamine derivatives, and many others. Some of these have been incorporated as HTMs in complete, functional PSCs capable of matching the performance of the best-performing PSCs prepared using spiro-OMeTAD while showing even better stability.

In light of these results, we describe the advances made in the synthesis of HTMs that have been tested in perovskite solar cells, and give an overview of the molecular engineering of HTMs. Meanwhile, we highlight the effects of molecular structure on PCE and device stability of PSCs. This review is organized as follows. In the first part, we give a general introduction to the development of PSCs. In the second part, we focus on the introduction of the perovskite structure, device architecture, and relevant work principles in detail. In the third part, we discuss all kinds of molecular HTMs applied in PSCs. Special emphasis is placed on the relationship between HTM molecular structure and device function. Last but not least, we point out some existing challenges, suggest possible routes for further HTM design, and provide some conclusions.

The construction of a photo-controllable artificial molecular machine capable of realizing the light-driven motion on a molecular scale and of performing a specific function is a fascinating topic in supramolecular chemistry. The bistable switchable molecule, azobenzene (AZO), has been introduced into the supramolecular architecture as a key building block, owing to its efficient and reversible trans (E)-cis (Z) photoisomerization. The binding strength of the dibenzo[24]crown-8 (DB24C8) host and dialkylammonium-based rod-like guest consisting of an AZO moiety and the Z$\to $E photoisomerization process in an interlocked host-guest complex have been investigated by the density functional theory (DFT) calculations and the reactive molecular dynamics (RMD) simulations by considering both torsion and inversion paths. The strong host-guest binding strength provides a necessary premise to stabilize the complex during the E-Z photoisomerization of the AZO unit, which is a terminal stopper to control the directional motion of the guest. A stronger binding strength for the Z isomer can be induced by the stronger hydrogen-bonding interaction. The steric effect is introduced into the Z isomer to force the ring slipping exclusively over the cyclopentyl terminal (pseudostopper). The host-guest complexation has a slight effect on the conformation of the AZO functional subunit for the two isomers. The faster Z$\to $E photoisomerization process within the picosecond timescale is kinetically more favored than the dethreading of the ring through the pseudostopper subunit of the rod. After isomerization, a structure relaxation is observed for the crown ether ring within 500 ps. The flexible backbone of the crown ether ring is helpful in realizing steady and stable host-guest recognition during photoisomerization. Moreover, the orthogonality of the site-specific binding interaction is revealed by the similar binding energies obtained at similar hydrogen bonding recognition sites for various interlocked host-guest supramolecular systems although the constituents of the guests are different from each other. The introduction of two stereoisomers of the AZO subunit has little influence on the other conformations of guest subunits. These results are useful for the rational design of more sophisticated stimuli-controlled artificial molecular machines.

Cu has been widely used as a substrate material for graphene growth. To understand the atomistic mechanism of growth, an efficient and accurate method for describing Cu-C interactions is necessary, which is the prerequisite of any possible large-scale molecular simulation studies. The semi-empirical density-functional tight-binding (DFTB) method has a solid basis from the density functional theory (DFT) and is believed to be a good tool for achieving a balance between efficiency and accuracy. However, existing DFTB parameters cannot provide a reasonable description of the Cu surface structure. At the same time, DFTB parameters for Cu-C interactions are not available. Therefore, it is highly desirable to develop a set of DFTB parameters that can describe the Cu-C system, especially for surface reactions. In this study, a parametrization for Cu-C systems within the self-consistent-charge DFTB (SCC-DFTB) framework is performed. One-center parameters, including on-site energy, Hubbard, and spin parameters, are obtained from DFT calculations on free atoms. Two-center parameters can be calculated based on atomic wavefunctions. The remaining repulsive potential is obtained as the best compromise to describe different kinds of systems. Test calculations on Cu surfaces and Cu-or C atom-adsorbed Cu surfaces indicate that the obtained parameters can generate reasonable geometric structures and energetics. Based on this parameter set, carbon dimerization on the Cu(111) surface has been investigated via molecular dynamics simulations. Since they are the feeding species for graphene growth, it is important to understand how carbon dimers are formed on the Cu surface. It is difficult to observe carbon dimerization in brute-force MD simulations even at high temperatures, because of the surface structure distortion. To study the dimerization mechanism, metadynamics simulations are performed. Our simulations suggest that carbon atoms will rotate around the bridging Cu atom after a bridging metal structure is formed, which eventually leads to the dimer formation. The free energy barrier for dimerization at 1300 K is about 0.9 eV. The results presented here provide useful insights for understanding graphene growth.

Graphdiyne features sp and sp² hybridized carbon atoms. The direct natural band gap and Dirac cone structure for graphdiyne are believed to originated from inhomogeneous π-bonding of differently hybridized carbon atoms and overlap of carbon 2p_z orbitals. The special electronic structures and pore structures of graphdiyne are responsible for its potential and important applications in the fields of information technology, electronics, energy, catalysis, and optoelectronics. Recent basic and applied research studies of graphdiyne have led to important results; as a result, graphdiyne has become a new research field for carbon materials. The high activity of acetylenic units in graphdiyne provides a good platform for chemical modification and doping. Several approaches have been developed to modify the band gap of graphdiyne, including invoking strain, BN-doping, preparing nanoribbons, and hydrogenation, leading to a new graphdiyne (GDY) or graphyne (GY) derivatives. In this review, we summarize the recent progress in nonmetallic heteroatom doping, especially by nitrogen, boron, or oxygen; by modifying metal atoms for tuning electronic/spintronic properties, enhancing water splitting performance, and applying dye-sensitized solar cells and catalysts; and by surface functionalization of graphdiyne via hydrogenation, hydroxylation, and halogenation to adjust the band gap. Hence, it can be surmised that the electronic structures of graphdiynes can be tuned for specific applications. These results suggest that graphdiynes can be more advantageous than grapheme for tailoring energy band gaps for application in nanoelectronics. We also discuss the influence of doping and functionalization on the electronic properties of graphdiyne and their effects on the synergistic enhancement of photoelectrocatalytic performance. We hope that the deep and wide application of these new materials in many fields such as energy transfer and storage, catalyst, electronics, gas separation, and spintronics will draw much attention and become a widely focused research direction.

Graphyne is a rapidly rising star material of carbon allotropes containing only sp and sp² hybridized carbon atoms forming extended two-dimensional layers. In particular, graphdiyne is an important member of graphyne family. With unique nanotopological pores, two-dimensional layered conjugated frameworks, and excellent semiconducting and optical properties, graphdiyne has displayed distinct superiorities in the fields of energy storage, electrocatalysis, photocatalysis, nonlinear optics, electronics, gas separation, etc. Therefore, the synthesis of high-quality graphdiyne is highly required to fulfill its potentially extraordinary applications. Furthermore, the development of a standardized and systematic set of characterization procedures is an urgent need, and would be based on intrinsic samples. However, there are still obvious barriers to synthesizing this new-born carbon allotrope that can be mainly considered as follows. The selection and stability of monomers is essential for synthesis. The synthesis process in solution also suffers from an annoying problem of the relatively free rotation possible about the alkyne-aryl single bonds, which leads to the coexistence and rapid equilibration of coplanar and twisted structures. Furthermore, the limited reaction conversion and side reactions also lead to a confusion of configuration.

In this review, we primarily focus on the state-of-the-art progress of the synthetic strategies for graphdiyne. First, we give a brief introduction about the structure of graphyne and graphdiyne. We subsequently discuss in detail the recent developments in synthetic methods that can mainly be divided into three aspects: total organic synthesis, on-surface covalent reaction, and polymerization in a solution phase. In particular, much progress in solution polymerization has been achieved since in-situ polymerization on Cu surface was reported in 2010. Liquid/liquid interface, gas/liquid interface, and surface template were also employed for confined reaction, and contribute significantly to the synthesis of a graphdiyne film. Through such strategies, graphdiyne with a well-defined structure and diverse morphologies could be achieved successfully. Finally, the opportunities and challenges for the synthesis of graphdiyne are prospected. A more rational design is desired in terms of monomer modification and reaction regulation.