Enhancement of short/medium-range order and thermal conductivity in ultrahard sp3 amorphous carbon by C70 precursor.

As an advanced amorphous material, sp3 amorphous carbon exhibits exceptional mechanical, thermal and optical properties, but it cannot be synthesized by using traditional processes such as fast cooling liquid carbon and an efficient strategy to tune its structure and properties is thus lacking. Here we show that the structures and physical properties of sp3 amorphous carbon can be modified by changing the concentration of carbon pentagons and hexagons in the fullerene precursor from the topological transition point of view. A highly transparent, nearly pure sp3-hybridized bulk amorphous carbon, which inherits more hexagonal-diamond structural feature, was synthesized from C70 at high pressure and high temperature. This amorphous carbon shows more hexagonal-diamond-like clusters, stronger short/medium-range structural order, and significantly enhanced thermal conductivity (36.3 ± 2.2 W m-1 K-1) and higher hardness (109.8 ± 5.6 GPa) compared to that synthesized from C60. Our work thus provides a valid strategy to modify the microstructure of amorphous solids for desirable properties.

Realizing long range π-conjugation in phenanthrene and phenanthrene-based molecular crystals for anomalous piezoluminescence.

Unlike the known aggregation-caused quenching (ACQ) that the enhancement of π-π interactions in rigid organic molecules usually decreases the luminescent emission, here we show that an intermolecular "head-to-head" π-π interaction in the phenanthrene crystal, forming the so-called "transannular effect", could result in a higher degree of electron delocalization and thus photoluminescent emission enhancement. Such a transannular effect is molecular configuration and stacking dependent, which is absent in the isomers of phenanthrene but can be realized again in the designed phenanthrene-based cocrystals. The transannular effect becomes more significant upon compression and causes anomalous piezoluminescent enhancement in the crystals. Our findings thus provide new insights into the effects of π-π interactions on luminescence emission and also offer new pathways for designing efficient aggregation-induced emission (AIE) materials to advance their applications.

Doping of charge-transfer molecules in cocrystals for the design of materials with novel piezo-activated luminescence.

A novel piezo-activated luminescent material with wide range modulation of the luminescence wavelength and a giant intensity enhancement upon compression was prepared using a strategy of molecular doping. The doping of THT molecules into TCNB-perylene cocrystals results in the formation of a weak but pressure-enhanced emission center in the material at ambient pressure. Upon compression, the emissive band from the undoped component TCNB-perylene undergoes a normal red shift and emission quenching, while the weak emission center shows an anomalous blue shift from 615 nm to 574 nm and a giant luminescence enhancement up to 16 GPa. Further theoretical calculations show that doping by THT could modify intermolecular interactions, promote molecular deformation, and importantly, inject electrons into the host TCNB-perylene upon compression, which contributes to the novel piezochromic luminescence behavior. Based on this finding, we further propose a universal approach to design and regulate the piezo-activated luminescence of materials by using other similar dopants.

Pressure-Dependent Structural and Band Gap Tuning of Semiconductor Copper(I) Thiocyanate (CuSCN).

Copper(I) thiocyanate (CuSCN) is a p-type semiconductor with exceptional properties for optoelectronic devices such as solar cells, thin-film transistors , organic light-emitting diodes, etc. Understanding the structure-optical property relationships in CuSCN is critical for its optoelectronic applications. Herein, high-pressure techniques combined with theoretical calculations are used to thoroughly investigate the structural and optical changes of CuSCN upon compression. Under high pressure, CuSCN exhibits a progressive decrease of the band gap with different rates, which is relevant to the ß to α phase transition in CuSCN and the subsequent amorphization through polymerization. UV-vis spectra measurements reveal a reduction in band gap from 3.4 to 1.3 eV upon decompression to ambient conditions. Such transitions could be attributed to the pressure-induced rotation of CuNS3 tetrahedron and bond length shrinkage. The severe distortion of the polyhedral units prompts breakdown of the structure and thus the amorphization, which is quenchable to ambient conditions. Our study demonstrates that high pressure can be utilized to adjust the structure and optical characteristics of CuSCN compound, potentially extending the material's uses in optoelectronic devices.

Anomalous pressure-responsive emission enhancement of FCO-CzS due to molecular configuration and electronic structure changes.

Studying the stimuli-responsive properties of luminescent materials is important for their applications, while the luminescent materials studied up to now usually exhibit emission quenching and red shift in photoluminescence (PL) energy upon compression. Designing luminescent material with abnormal pressure responses remains challenging. Here, we report the discovery of abnormal luminescent properties of FCO-CzS upon compression. A theoretical study on the excited state decay process has been carried out for FCO-CzS at high pressure by hybrid quantum mechanics/molecular mechanics (QM/MM). A significant emission enhancement and blue shift are observed as pressure increases up to 20 GPa. This is opposite to the pressure response behaviours reported for other luminescent materials. It is further revealed that both the unique molecular configuration and the electronic structure change contribute to the anomalous pressure-responsive emission of FCO-CzS, which reduces the non-radiative rate and increases the radiative rate, respectively. Our study provides a strategy for the design of luminescent materials with desired pressure responses.

Pressure-stabilized polymerization of nitrogen in manganese nitrides at ambient and high pressures.

Two stable high-pressure phases (C2/m-MnN4 and P1̄-MnN4) and four metastable phases (P4/mmm-MnN4, P1̄-MnN5, C2/m-MnN6 and P1̄-MnN8) are proposed by using ab initio evolutionary simulations. Besides the reported quasi-diatomic molecule N2, the armchair chain and S-like chain, the N4 ring and N22 ring are firstly reported in the P4/mmm-MnN4 and P1̄-MnN5 phases. A detailed study is performed on the energetic properties, mechanical properties and stability of these polynitrogen structures. Ab initio molecular dynamics simulations show that P1̄-MnN4 and P1̄-MnN5 can be quenched down to ambient conditions, and large decomposition energy barriers result in the high decomposition temperatures of P1̄-MnN4 (2000 K) and P1̄-MnN5 (3000 K). Interestingly, P4/mmm-MnN4 with the N4 ring exhibits outstanding mechanical properties, including high incompressibility, high hardness, uniform strength in the 2-D direction and excellent ductility. Strong N-N covalent bond and weak Mn-N ionic bond interactions are observed in the predicted Mn-N compounds, and the charge transfer between the Mn and N atoms provides an important contribution to the stabilization of polymeric N-structures. All the proposed structures are metallic phases. Our results provide a deep understanding of the chemistry of transition metal polynitrides under pressure and encourage experimental synthesis of these new manganese polynitrides in future.

Ultrahard bulk amorphous carbon from collapsed fullerene.

Amorphous materials inherit short- and medium-range order from the corresponding crystal and thus preserve some of its properties while still exhibiting novel properties1,2. Due to its important applications in technology, amorphous carbon with sp2 or mixed sp2-sp3 hybridization has been explored and prepared3,4, but synthesis of bulk amorphous carbon with sp3 concentration close to 100% remains a challenge. Such materials inherit the short-/medium-range order of diamond and should also inherit its superior properties5. Here, we successfully synthesized millimetre-sized samples-with volumes 103-104 times as large as produced in earlier studies-of transparent, nearly pure sp3 amorphous carbon by heating fullerenes at pressures close to the cage collapse boundary. The material synthesized consists of many randomly oriented clusters with diamond-like short-/medium-range order and possesses the highest hardness (101.9 ± 2.3 GPa), elastic modulus (1,182 ± 40 GPa) and thermal conductivity (26.0 ± 1.3 W m-1 K-1) observed in any known amorphous material. It also exhibits optical bandgaps tunable from 1.85 eV to 2.79 eV. These discoveries contribute to our knowledge about advanced amorphous materials and the synthesis of bulk amorphous materials by high-pressure and high-temperature techniques and may enable new applications for amorphous solids.

Molecular insertion regulates the donor-acceptor interactions in cocrystals for the design of piezochromic luminescent materials.

Developing a universal strategy to design piezochromic luminescent materials with desirable properties remains challenging. Here, we report that insertion of a non-emissive molecule into a donor (perylene) and acceptor (1,2,4,5-tetracyanobezene) binary cocrystal can realize fine manipulation of intermolecular interactions between perylene and 1,2,4,5-tetracyanobezene (TCNB) for desirable piezochromic luminescent properties. A continuous pressure-induced emission enhancement up to 3 GPa and a blue shift from 655 to 619 nm have been observed in perylene-TCNB cocrystals upon THF insertion, in contrast to the red-shifted and quenched emission observed when compressing perylene-TCNB cocrystals and other cocrystals reported earlier. By combining experiment with theory, it is further revealed that the inserted non-emissive THF forms blue-shifting hydrogen bonds with neighboring TCNB molecules and promote a conformation change of perylene molecules upon compression, causing the blue-shifted and enhanced emission. This strategy remains valid when inserting other molecules as non-emissive component into perylene-TCNB cocrystals for abnormal piezochromic luminescent behaviors.