Effects of strain and thickness on the mechanical, electronic, and optical properties of Cu2Te.

Two-dimensional transition-metal chalcogenides (TMCs) have attracted considerable attention because of their exceptional photoelectric properties, finding applications in diverse fields such as photovoltaics, lithium-ion batteries, catalysis, and energy conversion and storage. Recently, experimentally fabricated monolayers of semiconducting Cu2Te have emerged as intriguing materials with outstanding thermal and photoelectric characteristics. In this study, we employ first-principles calculations to investigate the mechanical, electronic, and optical properties of monolayer Cu2Te exhibiting both λ and ζ structures, considering the effects of thickness and strain. The calculations reveal the robust mechanical stability of λ-Cu2Te and ζ-Cu2Te under varying thickness and strain conditions. By applying -5% to +5% strain, the band gaps can be modulated, with ζ-Cu2Te exhibiting an indirect-to-direct transition at a biaxial strain of +5%. In addition, a semiconductor-to-metal transition is observed for both ζ-Cu2Te and λ-Cu2Te with increasing thickness. The absorption spectra of λ-Cu2Te and ζ-Cu2Te exhibit a redshift with an increase in the number of layers. These computational insights into Cu2Te provide valuable information for potential applications in nano-electromechanical systems, optoelectronics, and photocatalytic devices and may guide subsequent experimental research efforts.

Modulation of supramolecular structure by stepwise removal of tert-butyl groups from tetraazaperopyrene derivatives on Ag(111).

Tert-butyl functional groups can modulate the self-assembly behavior of organic molecules on surfaces. However, the precise construction of supramolecular architectures through their controlled thermal removal remains a challenge. Herein, we precisely controlled the removal amount of tert-butyl groups in tetraazaperopyrene derivatives by stepwise annealing on Ag(111). The evolution of 4tBu-TAPP supramolecular self-assembly from the grid-like structure composed of 3tBu-TAPP through the honeycomb network formed by 2tBu-TAPP to the one-dimensional chain co-assembled by tBu-TAPP and TAPP was successfully realized. This series of supramolecular nanostructures were directly visualized by high resolution scanning tunneling microscopy. Tip manipulation and density functional theory calculations show that the formation of honeycomb network structure can be attributed to the van der Waals interactions, N-Ag-N coordination bonds, and weak C-H⋯N hydrogen bonds. Further addition of two tert-butyl groups (6tBu-TAPP) leads to a completely different assembly evolution, due to the fact that the additional tert-butyl groups affect the molecular adsorption behavior and ultimately induce desorption. This work can possibly be exploited in constructing stable and long-range ordered nanostructures in surface-assisted systems, which can also promote the development of nanostructures in functional molecular devices.

Adsorption configurations and electronic properties of self-assembled C60and C70molecules on a semiconductor CuSe monolayer with periodic nanopores.

Two-dimensional (2D) supramolecular self-assembly architectures are considered one of the most significant and challenging topics in nanotechnology and modern organic chemistry. The study of these processes on surfaces is vital to achieving a higher degree of control in the design of supramolecular architecture. Herein, we report on the 2D self-assembly monolayer architectures based on C60and C70molecules on a semiconductor CuSe monolayer with periodic nanopores, which are essential for providing ideas for surface template chemistry. With the aid of low-temperature scanning tunneling microscopy/spectroscopy (LT-STM/STS) and density functional theory (DFT) calculation methods, we systematically investigate the adsorption configurations and electronic properties of C60and C70on CuSe monolayer with periodic nanopores. Our results show that both the C60and C70molecules above the nanopores will fall into the nanopores, while those on the CuSe surface will show well-defined self-assembly with various adsorption configurations. Besides, through STS measurement, the lowest unoccupied molecular orbitals (LUMOs) and characteristic peaks of fullerene molecules will be slightly different due to different adsorption configurations. This work helps us to study the adsorption behavior of the fullerene family on various kinds of semiconductor substrates, and also provides vigorous support for the development of fullerene electrical devices in the future.

On-surface synthesis of Au-C4 and Au-O4 alternately arranged organometallic coordination networks via selective aromatic C-H bond activation.

Selective activation of the C-H bond of aromatic hydrocarbons is significant in synthetic chemistry. However, achieving oriented C-H activation remains challenging due to the poor selectivity of aromatic C-H bonds. Herein, we successfully constructed alternately arranged Au-C4 and Au-O4 organometallic coordination networks through selective aromatic C-H bond activation on Au(111) substrate. The stepwise reaction process of the 5, 12-dibromopyrene 3,4,9, 10-tetracarboxylic dianhydride precursor is monitored by high-resolution scanning tunneling microscopy. Our results show that the gold atoms in C-Au-C organometallic chains play a crucial role in promoting the selective ortho C-H bonds activation and forming Au-C4 coordination structure, which is further demonstrated by a comparative experiment of PTCDA precursor on Au(111). Furthermore, our experiment of 2Br-PTCDA precursor on Cu(111) substrate confirms that copper atoms in C-Cu-C organometallic chains can also assist the formation of Cu-C4 coordination structure. Our results reveal the vital effect of organometallic coordination on selective C-H bond activation of reactants, which holds promising implications for controllable on-surface synthesis.

On-surface synthesis of two types of cyano-substituted polyfluorene derivatives via Ullmann coupling on Au(111).

Using 4-(3,6-dibromo-9H-carbazol-9-yl)benzonitrile (DBCB) precursors, we successfully constructed two types of cyano-substituted polymers on Au(111) by the molecular beam epitaxy method. According to the geometry, the two polymers are referred to as w-type polymers composed of cis-dimers and z-type polymers composed of trans-dimers. The intermediate dimers and final polymers were well characterized by high-resolution scanning tunneling microscopy (HR-STM). Moreover, the productivities of these two polymers can be controlled by adjusting the heating rate and different treatment methods. High heating rates and hot deposition can provide more ample space and time for molecular diffusion, which is conducive to the formation of w-type polymers with relatively low density. In addition, by combining scanning tunneling spectroscopy (STS) and density functional theory (DFT) calculations, we have shown that the addition of CN groups reduces the band gap of the two polymers. Our investigation thus shows the controllable construction of nanostructures through efficient surface synthesis parameters and reveals the potential of using functional groups as tools to modify the electronic properties of polymers.