Shengzhong Liu

Fullerenes as precursors for diamond film growth without hydrogen or oxygen additions

Diamond films are predominantly grown using approximately 1% of a hydrocarbon precursor in hydrogen gas. Hydrogen is generally believed to be necessary for the diamond thin‐film growth process. However, hydrogen in varying amounts is inevitably incorporated in the growing diamond lattice, leading to structural defects. We report here the successful growth of diamond films using fullerene precursors in an argon microwave plasma, a unique development achieved without the addition of hydrogen or oxygen. We speculate that collisional fragmentation of C60 to give C2 could be responsible for the high growth rate of the very‐fine‐grained diamond films.

The structure of the C60 molecule: X-ray crystal structure determination of a twin at 110 K

Single-crystal x-ray diffraction methods were used to determine the crystal and molecular structure of C60 buckminsterfullerene. At 110 kelvin C60 is cubic, apparent Laue symmetry m3m, but it exhibits noncrystallographic systematic extinctions indicative of a twin in which I(hkl) and I(khl) are superimposed. In fact, C60 crystallizes with four molecules in space group [See equation in the PDF file] of the cubic system (Laue symmetry m3) with lattice constant a = 14.052(5) angstroms (�) at 110 kelvin. The twin components are equal. A given component, which has crystallographically imposed symmetry [See equation in the PDF file] displays an ordered structure of a truncated icosahedron. The five independent C=C bonds that join C6 rings average 1.355(9) �; the ten independent C—C bonds that join C6 and C5 rings average 1.467(21) �. The mean atom-to-atom diameter of the C60 molecule is 7.065(3) �. The molecules are very tightly packed in the crystal structure, with intermolecular C...C distances as short as 3.131(7) �.

High efficiency flexible perovskite solar cells using superior low temperature TiO2

A process is developed to prepare a very dense TiO2 layer using magnetron sputtering at room temperature. It is found that the film is amorphous in nature, offering faster electron transport, reduced transfer resistance and better performance for electron extraction from the perovskite absorber layer. It is these superior electronic properties that makes it possible for us to achieve 15.07% efficiency flexible perovskite solar cells, on a respectably large area >10 mm2. It is the highest efficiency reported to date for the flexible perovskite devices.

Surface optimization to eliminate hysteresis for record efficiency planar perovskite solar cells

The electron-transport layer (ETL) between the active perovskite material and the cathode plays a critical role in planar perovskite solar cells. Herein, we report a drastically improved solar cell efficiency via surface optimization of the TiO2 ETL using a special ionic-liquid (IL) that shows high optical transparency and superior electron mobility. As a consequence, the efficiency is promoted to as high as 19.62% (the certified efficiency is 19.42%), exceeding the previous highest efficiency recorded for planar CH3NH3PbI3 perovskite solar cells. Surprisingly, the notorious hysteresis is completely eliminated, likely due to the improved ETL quality that has effectively suppressed ion migration in the perovskite layer and charge accumulation at the interfaces, higher electron mobility to balance the hole flux at the anode, and a better growth platform for the high quality perovskite absorber. Both experimental analyses and theoretical calculations reveal that the anion group of the IL bonds to TiO2, leading to a higher electron mobility and a well-matched work function. Meanwhile, the cation group interfaces with adjacent perovskite grains to provide an effective channel for electron transport and a suitable setting to grow low trap-state density perovskite for improved device performance.

Buckyball microwave plasmas: Fragmentation and diamond-film growth

Microwave discharges (2.45 GHz) have been generated in C60‐containing Ar. The gas mixtures were produced by flowing Ar over fullerene‐containing soot at a variety of temperatures. Optical spectroscopy shows that the spectrum is dominated by the d 3Πg–a  3Πu Swan bands of C2 and particularly the Δv=−2, −1, 0, +1, and +2 sequences. These results give direct evidence that C2 is in fact one of the products of C60 fragmentation brought about, at least in part, by collisionally induced dissociation. C60 has been used as a precursor in a plasma‐enhanced chemical vapor deposition experiment to grow diamond‐thin films. The films, grown in an Ar/H2 gas mixture (0.14% carbon content, 100 Torr, 20 sccm Ar, 4 sccm H2, 1500 W, 850 °C substrate temperature) were characterized with scanning electron microscopy, x‐ray diffraction, and Raman spectroscopy. The growth rate was found to be ∼0.6 μm/h. Assuming a linear dependence on carbon concentration, a growth rate at least six times higher than commonly observed using meth...

One-step hydrothermal synthesis of monolayer MoS2 quantum dots for highly efficient electrocatalytic hydrogen evolution

Designing MoS2 nanocatalysts with ultrasmall size and few layers is an effective strategy to enhance their electrocatalytic activity in the hydrogen evolution reaction (HER). In this work, uniform MoS2 quantum dots (QDs) are synthesized by a novel and facile hydrothermal route. Transmission electron microscopy and atomic force microscopy measurements show that the resulting products possess a monolayer thickness with an average size of about 3.6 nm. Benefiting from the abundance of exposed catalytic edge sites, as well as the excellent intrinsic conductivity of the monolayer structure, the MoS2 QDs showed excellent catalytic activity with a low overpotential of approximately 160 mV and a small Tafel slope of 59 mV dec−1, which made it a promising HER electrocatalyst for practical applications.

Nucleation of diamond films on surfaces using carbon clusters

A unique method for nucleating diamondfilms on surfaces using C clusters is described. The process substitutes the need for diamond polish pretreatment of substrates prior to diamondfilm growth, as currently practiced in low‐pressure (<1 atm) chemical vapor deposition methods. As an example, the use of C clusters C60 and C70 as nucleating layers on single‐crystal Si surfaces is presented. It is shown that a thin layer (approximately 1000 A) of pure carbon C70 is sufficient for the nucleation and growth of fine grain polycrystallinediamondfilms. The enhancement of nucleation by the C70 layer is nearly ten orders of magnitude over an untreated Si surface. It also follows that C clusters can be used as a one‐step lithographic template for growingdiamond on selected regions of the substrate. In addition, insight into the mechanism for diamondnucleation from C clusters is given.

Deposition and characterization of nanocrystalline diamond films

Highly uniform, smooth nanocrystalline diamond films have been fabricated with a magnetoactive microwave chemical vapor deposition system. The top and bottom magnet currents were 145 and 60 A, respectively, while the microwave power and substrate temperature were controlled at 1500 W and 850 °C, respectively during deposition. The total processing pressure was regulated at 40 Pa (300 mTorr) with gas‐flow rates of 30 sccm of hydrogen, 2.4 sccm of methane, and 1 sccm of oxygen. Diamond films obtained under these conditions have grain sizes between 0.1 and 0.3 μm, and a mean roughness of 14.95 nm. The growth rate is 0.1 μm/h. Characterization techniques have involved x‐ray diffraction, Raman spectroscopy, scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. Both x‐ray and electron diffraction patterns show no evidence of graphitic phase. Although a high density of twins and stacking faults was revealed by high‐resolution electron microscopy, compact diamond grains, and...

Infrared and Raman spectra of C60 and C70 solid films at room temperature

Infrared and high‐resolution Raman spectroscopic probes of thin films of C60 and C70 are presented and discussed in terms of previous measurements, semi‐empirical calculations, and plausible molecular geometries.

Stable high efficiency two-dimensional perovskite solar cells via cesium doping

Two-dimensional (2D) organic–inorganic perovskites have recently emerged as one of the most important thin-film solar cell materials owing to their excellent environmental stability. The remaining major pitfall is their relatively poor photovoltaic performance in contrast to 3D perovskites. In this work we demonstrate cesium cation (Cs+) doped 2D (BA)2(MA)3Pb4I13 perovskite solar cells giving a power conversion efficiency (PCE) as high as 13.7%, the highest among the reported 2D devices, with excellent humidity resistance. The enhanced efficiency from 12.3% (without Cs+) to 13.7% (with 5% Cs+) is attributed to perfectly controlled crystal orientation, an increased grain size of the 2D planes, superior surface quality, reduced trap-state density, enhanced charge-carrier mobility and charge-transfer kinetics. Surprisingly, it is found that the Cs+ doping yields superior stability for the 2D perovskite solar cells when subjected to a high humidity environment without encapsulation. The device doped using 5% Cs+ degrades only ca. 10% after 1400 hours of exposure in 30% relative humidity (RH), and exhibits significantly improved stability under heating and high moisture environments. Our results provide an important step toward air-stable and fully printable low dimensional perovskites as a next-generation renewable energy source.