Site-Recognition-Induced Structural and Photoluminescent Evolution of the Gold-Pincer Nanocluster.

The induced structural transformation provides an efficient way to precisely modulate the fine structures and the corresponding performance of gold nanoclusters, thus constituting one of the important research topics in cluster chemistry. However, the driving forces and mechanisms of these processes are still ambiguous in many cases, limiting further applications. In this work, based on the unique coordination mode of the pincer ligand-stabilized gold nanocluster Au8(PNP)4, we revealed the site-recognition mechanism for induced transformations of gold nanoclusters. The "open nitrogen sites" on the surface of the nanocluster interact with different inducers including organic compounds and metals and trigger the conversion of Au8(PNP)4 to Au13 and Au9Ag4 nanoclusters, respectively. Control experiments verified the site-recognition mechanism, and the femtosecond and nanosecond transient absorption spectroscopy revealed the electronic and photoluminescent evolution accompanied by the structural transformation.

Controlled Sequential Doping of Metal Nanocluster.

Atomically precise doping of metal nanoclusters provides excellent opportunities not only for subtly tailoring their properties but also for in-depth understanding of composition (structure)-property correlation of metal nanoclusters and has attracted increasing interest partly due to its significance for fundamental research and practical applications. Although single and multiple metal atom doping of metal nanoclusters (NCs) has been achieved, sequential single-to-multiple metal atom doping is still a big challenge and has not yet been reported. Herein, by introducing a second ligand, a novel multistep synthesis method was developed, controlled sequential single-to-multiple metal atom doping was successfully achieved for the first time, and three doped NCs Au25Cd1(p-MBT)17(PPh3)2, Au18Cd2(p-MBT)14(PPh3)2, and [Au19Cd3(p-MBT)18]- (p-MBTH: para-methylbenzenethiol) were obtained, including two novel NCs that were precisely characterized via mass spectrometry, single-crystal X-ray crystallography, and so forth. Furthermore, sequential doping-induced evolutions in the atomic and crystallographic structures and optical and catalytic properties of NCs were revealed.

Building Block Metal Nanocluster-Based Growth in 1D Direction.

Metal nanoclusters with precisely modulated structures at the nanoscale give us the opportunity to synthesize and investigate 1D nanomaterials at the atomic level. Herein, it realizes selective 1D growth of building block nanocluster "Au13 Cd2 " into three structurally different nanoclusters: "hand-in-hand" (Au13 Cd2 )2 O, "head-to-head" Au25 , and "shoulder-to-shoulder" Au33 . Detailed studies further reveals the growth mechanism and the growth-related tunable properties. This work provides new hints for the predictable structural transformation of nanoclusters and atomically precise construction of 1D nanomaterials.

Photochemical Route for Synthesizing Atomically Precise Metal Nanoclusters from Disulfide.

Practical approaches to the synthesis of atomically precise metal nanoclusters are in high demand as they provide the structural basis for investigating nanomaterials' structure-property correlations with atomic precision. The Brust-Schiffrin method has been widely used, while the essential reductive ligands (e.g., thiols) limit the application of this method for synthesizing metal nanoclusters with specific frameworks and surface ligands. In this work, we developed a photochemical route for synthesizing atomically precise metal nanoclusters by applying disulfide, which is a widely available, stable, and environmentally friendly sulfur source. This method enables the construction of structurally diverse metal nanoclusters and especially features the synthesis of PhS-protected metal nanoclusters that were not easily achieved previously and the gram-scale synthesis. A reduction-oxidation cascade mechanism has been revealed for the photochemical route. This work is expected to open up new opportunities for metal nanocluster synthesis and will contribute to the practical applications of this kind of nanomaterial.

Asymmetric transformation of achiral gold nanoclusters with negative nonlinear dependence between chiroptical activity and enantiomeric excess.

The investigation of chirality at the nanoscale is important to bridge the gap between molecular and macroscopic chirality. Atomically precise metal nanoclusters provide an ideal platform for this research, while their enantiopure preparation poses a challenge. Here, we describe an efficient approach to enantiopure metal nanoclusters via asymmetric transformation, that is, achiral Au23(SC6H11)16 nanoclusters are converted into chiral and enantiopure Au24(L)2(SC6H11)16 nanoclusters by a chiral inducer phosphoramidite (L). Two enantiomers of Au24(L)2(SC6H11)16 are obtained and the crystal structures reveal their hierarchical chirality, which originates from the two introduced chiral L molecules, the transformation-triggered asymmetric rearrangement of the staple motifs on the surface of the gold core, and the helical arrangement of nanocluster molecules. The construction of this type of enantiomerically pure nanoclusters is achieved based on the easy-to-synthesize and modular L. Lastly, the chirality-related chiroptical performance was investigated, revealing a negative nonlinear CD-ee dependence.

Preorganized Nitrogen Sites for Au11 Amidation: A Generalizable Strategy toward Precision Functionalization of Metal Nanoclusters.

Atomically precise metal nanoclusters have received tremendous attention due to their unique structures and properties. Although synthetic approaches to this kind of nanomaterial have been well developed, methods toward precision functionalization of the as-synthesized metal nanoclusters are extremely limited, hindering their interfacial modification and related performance improvement. Herein, an amidation strategy has been developed for the precision functionalization of the Au11 nanocluster based on preorganized nitrogen sites. The nanocluster amidation did not change the number of gold atoms in the Au11 kernel and their bonding mode to the surface ligands but slightly modified the arrangement of gold atoms with the introduction of functionality and chirality, thus representing a relatively mild method for the modification of metal nanoclusters. The stability and oxidation barrier of the Au11 nanocluster are also improved accordingly. The method developed here would be a generalizable strategy for the precision functionalization of metal nanoclusters.

Open Nitrogen Site-Induced Kinetic Resolution and Catalysis of a Gold Nanocluster.

The emerging metal nanocluster provides a platform for the investigation of structural features, unique properties, and structure-property correlation of nanomaterials at the atomic level. Construction of open sites on the surface of the metal nanocluster is a long-pursued but challenging goal. Herein, we realized the construction of "open organic sites" in a metal nanocluster for the first time. Specifically, we introduce the PNP (2,6-bis(diphenylphosphinomethyl)pyridine) pincer ligand in the synthesis of the gold nanocluster, enabling the construction of a structurally precise Au8(PNP)4 nanocluster. The rigidity and the unique bonding mode of PNP lead to open nitrogen sites on the surface of the Au8(PNP)4 nanocluster, which have been utilized as multifunctional sites in this work for efficient kinetic resolution and catalysis. The gold pincer nanocluster and the open nitrogen site-induced performance will be enlightening for the construction of multifunctional metal nanoclusters.

Structure and assembly of a hexanuclear AuNi bimetallic nanocluster.

An Au4Ni2 nanocluster containing a square-planar [-PPh2-Au-S-Au-]2 ring and two nickel-pincer arms is reported here. Abundant intra- and inter-cluster noncovalent interactions promote the assembly of the nanocluster into a porous framework material. The assembly-dependent unique solubility and photoluminescence were also investigated.

Understanding and controlling the efficiency of Au24M(SR)18 nanoclusters as singlet-oxygen photosensitizers.

Singlet oxygen, 1O2, can be generated by molecules that upon photoexcitation enable the 3O2 → 1O2 transition. We used a series of atomically precise Au24M(SR)18 clusters, with different R groups and doping metal atoms M. Upon nanosecond photoexcitation of the cluster, 1O2 was efficiently generated. Detection was carried out by time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The resulting TREPR transient yielded the 1O2 lifetime as a function of the nature of the cluster. We found that: these clusters indeed generate 1O2 by forming a triplet state; a more positive oxidation potential of the molecular cluster corresponds to a longer 1O2 lifetime; proper design of the cluster yields results analogous to those of a well-known reference photosensitizer, although more effectively. Comprehensive kinetic analysis provided important insights into the mechanism and driving-force dependence of the quenching of 1O2 by gold nanoclusters. Understanding on a molecular basis why these molecules may perform so well in 1O2 photosensitization is instrumental to controlling their performance.

Metal Doping of Au25(SR)18- Clusters: Insights and Hindsights.

The study of the structures and properties of atomically precise gold nanoclusters is the object of active research worldwide. Recently, research has been also focusing on the doping of metal nanoclusters through introduction of noble metals, such as platinum, and less noble metals, such as cadmium and mercury. Previous studies, which relied extensively on the use of mass spectrometry and single-crystal X-ray crystallography, led to the assignment of the location of each of these foreign-metal atoms. Our study provides new insights into this topic and, particularly, compelling evidence about the actual position of the selected metal atoms M = Pt, Pd, Hg, and Cd in the structure of Au24M(SR)180. To make sure that the results were not dependent on the thiolate, for SR we used both butanethiolate and phenylethanethiolate. The clusters were prepared according to different literature procedures that were supposed to lead to different doping positions. Use of NMR spectroscopy and isotope effects, with the support of mass spectrometry, electrochemistry, and single-crystal X-ray crystallography, led us to confirm that noble metals indeed dope the cluster at its central position, whereas no matter how the doping reaction is conducted and the nature of the ligand, the position of both Cd and Hg is always on the icosahedron shell, rather than at the central or staple position, as often reported. Our results not only provide a reassessment of previous conclusions, but also highlight the importance of NMR spectroscopy studies and cast doubts on drawing conclusions mostly based on single-crystal X-ray crystallography.

Heterogeneous graphene oxide membrane for rectified ion transport.

Ion transport in nanoconfinement has drawn significant attention due to its crucial role in the functioning of biological nanochannels and in the stimulation of applications including iontronics, biosensing and energy conversion. Graphene oxide (GO) membranes that contain abundant two-dimensional nanochannels formed in-between stacked GO nanosheets are particularly attractive because they offer high tunability in terms of channel dimensions and surface properties. However, because of the inherent homogeneity of the GO membrane, ion transport through such nanochannels typically exhibit ohmic behaviour, inhibiting its potential widespread applications. Herein, we demonstrate heterogeneous GO membranes for a voltage-driven ion transport. The membrane is composed of a negatively-charged GO and a positively-charged PEI-grafted GO laminate. Highly rectified ion transport are observed through such membranes. Molecular dynamics simulations are employed to reveal micro-processes of ion behaviours in the two-dimensional nanochannels of the heterogeneous membranes. Furthermore, an enhancement of rectification performance is achieved when charge asymmetry of nanochannels is strengthened by adjusting the pH conditions of the electrolyte solutions. Our study should provide a potential paradigm for the application of GO membranes in ion transport control and the use as ionic rectifiers.

Emerging hydrovoltaic technology.

Water contains tremendous energy in a variety of forms, but very little of this energy has yet been harnessed. Nanostructured materials can generate electricity on interaction with water, a phenomenon that we term the hydrovoltaic effect, which potentially extends the technical capability of water energy harvesting and enables the creation of self-powered devices. In this Review, starting by describing fundamental properties of water and of water-solid interfaces, we discuss key aspects pertaining to water-carbon interactions and basic mechanisms of harvesting water energy with nanostructured materials. Experimental advances in generating electricity from water flows, waves, natural evaporation and moisture are then reviewed to show the correlations in their basic mechanisms and the potential for their integration towards harvesting energy from the water cycle. We further discuss potential device applications of hydrovoltaic technologies, analyse main challenges in improving the energy conversion efficiency and scaling up the output power, and suggest prospects for developments of the emerging technology.

Ultrathin Molybdenum Dioxide Nanosheets as Uniform and Reusable Surface-Enhanced Raman Spectroscopy Substrates with High Sensitivity.

Metal oxides have advantages over the traditional noble metals to be used as substrate materials for surface-enhanced Raman spectroscopy (SERS) with low cost, versatility, and biocompatibility, but their enhancement factors are generally quite low with a poor limit of detection. Here, ultrathin molybdenum dioxide (MoO2 ) nanosheets synthesized by chemical vapor deposition demonstrated in large area are used as SERS substrates with superior signal uniformity in the whole area with a limit of detectable concentration down to 4 × 10-8 m and enhancement factor up to 2.1 × 105 , exceeding that of 2D materials and comparable to that of noble metal films. More practically important, the planar MoO2 substrate is more robust than noble metals and shows excellent reusability and uniformity, which is usually prohibited for nanostructured or nanoparticle-based metal oxide substrates. The enhancement is mainly attributed to the surface plasmon resonance effect as evidenced by the first principle calculations and UV-vis absorption spectroscopy characterization, which can be further increased by decreasing the thickness of the MoO2 nanosheets. The overall superior performance makes the MoO2 nanosheets an ideal substrate for practical SERS applications.

Molecular Beam Epitaxy of Highly Crystalline MoSe2 on Hexagonal Boron Nitride.

Molybdenum diselenide (MoSe2) is a promising two-dimensional material for next-generation electronics and optoelectronics. However, its application has been hindered by a lack of large-scale synthesis. Although chemical vapor deposition (CVD) using laboratory furnaces has been applied to grow two-dimensional (2D) MoSe2 cystals, no continuous film over macroscopically large area has been produced due to the lack of uniform control in these systems. Here, we investigate the molecular beam epitaxy (MBE) of 2D MoSe2 on hexagonal boron nitride (hBN) substrate, where highly crystalline MoSe2 film can be grown with electron mobility ∼15 cm2/(V s). Scanning transmission electron microscopy (STEM) shows that MoSe2 grains grown at an optimum temperature of 500 °C are highly oriented and coalesced to form continuous film with predominantly mirror twin boundaries. Our work suggests that van der Waals epitaxy of 2D materials is tolerant of lattice mismatch but is facilitated by substrates with similar symmetry.

Gold Fusion: From Au25(SR)18 to Au38(SR)24, the Most Unexpected Transformation of a Very Stable Nanocluster.

The study of the molecular cluster Au25(SR)18 has provided a wealth of fundamental insights into the properties of clusters protected by thiolated ligands (SR). This is also because this cluster has been particularly stable under a number of experimental conditions. Very unexpectedly, we found that paramagnetic Au25(SR)180 undergoes a spontaneous bimolecular fusion to form another benchmark gold nanocluster, Au38(SR)24. We tested this reaction with a series of Au25 clusters. The fusion was confirmed and characterized by UV-vis absorption spectroscopy, ESI mass spectrometry, 1H and 13C NMR spectroscopy, and electrochemistry. NMR evidences the presence of four types of ligand and, for the same proton type, double signals caused by the diastereotopicity arising from the chirality of the capping shell. This effect propagates up to the third carbon atom along the ligand chain. Electrochemistry provides a particularly convenient way to study the evolution process and determine the fusion rate constant, which decreases as the ligand length increases. No reaction is observed for the anionic clusters, whereas the radical nature of Au25(SR)180 appears to play an important role. This transformation of a stable cluster into a larger stable cluster without addition of any co-reagent also features the bottom-up assembly of the Au13 building block in solution. This very unexpected result could modify our view of the relative stability of molecular gold nanoclusters.

Water-evaporation-induced electricity with nanostructured carbon materials.

Water evaporation is a ubiquitous natural process that harvests thermal energy from the ambient environment. It has previously been utilized in a number of applications including the synthesis of nanostructures and the creation of energy-harvesting devices. Here, we show that water evaporation from the surface of a variety of nanostructured carbon materials can be used to generate electricity. We find that evaporation from centimetre-sized carbon black sheets can reliably generate sustained voltages of up to 1 V under ambient conditions. The interaction between the water molecules and the carbon layers and moreover evaporation-induced water flow within the porous carbon sheets are thought to be key to the voltage generation. This approach to electricity generation is related to the traditional streaming potential, which relies on driving ionic solutions through narrow gaps, and the recently reported method of moving ionic solutions across graphene surfaces, but as it exploits the natural process of evaporation and uses cheap carbon black it could offer advantages in the development of practical devices.

Layer Identification of Colorful Black Phosphorus.

A quick method for estimating the layer number of black phosphorus is demonstrated by simple color-comparison using optical microscope in this paper. A thickness-dependent reflection model of black phosphorus/SiO2 /Si is constructed and a colorbar confirmed by experiments is obtained for quick identifying layer number. The enhanced visibility affected by substrates or wavelength of light is further verified by calculating the contrast.

Low-voltage Driven Graphene Foam Thermoacoustic Speaker.

A low-voltage driven thermoacoustic speaker is fabricated based on three-dimensional graphene foams synthesized by a nickel-template assisted chemical vapor deposition method. The corresponding thermoacoustic performances are found to be related to its microstructure. Graphene foams exhibit low heat-leakage to substrates and feasible tunability in structures and thermoacoustic performances, having great promise for applications in flexible or ultrasonic acoustic devices.

1,4-Dibut-oxy-2,5-bis-{(Z)-2-[4-(9H-carbazol-9-yl)phen-yl]ethen-yl}benzene.

The title compound, C(54)H(48)N(2)O(2), lies about an inversion centre. The carbazole ring system makes dihedral angles of 58.43 (7) and 88.96 (7)°, respectively, with the adjacent and central benzene rings. The dihedral angle between the two benzene rings is 52.01 (8)°. In the crystal, mol-ecules are linked by pairs of C-H⋯O inter-actions, forming a tape along the a axis. The methyl group is disordered over two sets of sites with occupancies of 0.63 (3) and 0.37 (3).