arXiv subjects

Qiong Chen

Publications and source records attributed to Qiong Chen.

23 records · Page 2Linked to original sources

arXiv2017

Microscopic structure differences in CZTSe quaternary alloys prepared by different techniques revealed by spatially-resolved laser-induced-modification Raman spectroscopy

While producing comparable efficiencies and showing similar properties when probed by conventional techniques, such as Raman, photoluminescence and X-ray diffraction, two thin film solar cell materials with complex structures, such as quaternary compound CZTSe, may in fact differ significantly in their microscopic structures. In this work, laser induced modification Raman spectroscopy, coupled with high spatial resolution and high temperature capability, is demonstrated as an effective tool to obtain important structure information beyond that the conventional characterization techniques can offer, and thus to reveal the microscopic scale variations between nominally similar alloys. Specifically, CZTSe films prepared by sputtering and co-evaporation methods that exhibited similar Raman and XRD features were found to behave very differently under high laser power and high temperature Raman probe, because the differences in their microscopic structures lead to different structure modifications in response to the external stimuli, such as light illumination and temperature. They were also shown to undergo different degree of plastic changes and have different thermal conductivities as revealed by spatially-resolved Raman spectroscopy.

cond-mat.mtrl-sci↗

arXiv2017

Blueprint for nanoscale NMR

Nitrogen vacancy (NV) centers in diamond have been used as ultrasensitive magnetometers to perform nuclear magnetic resonance (NMR) spectroscopy of statistically polarized samples at 1 - 100 nm length scales. However, the spectral linewidth is typically limited to the kHz level, both by the NV sensor coherence time and by rapid molecular diffusion of the nuclei through the detection volume which in turn is critical for achieving long nuclear coherence times. Here we provide a blueprint for a set-up that combines a sensitivity sufficient for detecting NMR signals from nano- to micron-scale samples with a spectral resolution that is limited only by the nuclear spin coherence, i.e. comparable to conventional NMR. Our protocol detects the nuclear polarization induced along the direction of an external magnetic field with near surface NV centers using lock-in detection techniques to enable phase coherent signal averaging. Using NV centers in a dual role of NMR detector and optical hyperpolarization source to increase signal to noise, and in combination with Bayesian interference models for signal processing, nano/microscale NMR spectroscopy can be performed on sub-millimolar sample concentrations, several orders of magnitude better than the current state of the art.

quant-ph↗

arXiv2016

Multiple-stage structure transformation of organic-inorganic hybrid perovskite CH3NH3PbI3

By performing spatially resolved Raman and photoluminescence spectroscopy with varying excitation wavelength, density, and data acquisition parameters, we have achieved a unified understanding towards the spectroscopy signatures of the organic-inorganic hybrid perovskite, transforming from the pristine state (CH3NH3PbI3) to fully degraded state (i.e., PbI2) for samples with varying crystalline domain size from mesoscopic scale (approximately 100 nm) to macroscopic size (cm), synthesized by three different techniques. We show that the hybrid perovskite exhibits multiple stages of structure transformation occurring either spontaneously or under light illumination, with exceptionally high sensitivity to the illumination conditions (e.g., power, illumination time and interruption pattern). We highlight four transformation stages (Stage 1 - 4, with Stage 1 being the pristine state) along a primary structure degradation path exhibiting distinctly different Raman spectroscopy features at each stage, and point out that previously reported Raman spectra in the literature reflect degraded structures of either Stage 3 or 4. Additional characteristic optical features of partially degraded materials under the joint action of spontaneous and photo degradation are given. This study offers reliable benchmark results for understanding the intrinsic material properties and structure transformation of this unique category of hybrid materials, and a straightforward method to monitor the structure degradation after the material is used in a device or characterized by other techniques. The findings are pertinently important to a wide range of potential applications where the hybrid material is expected to function in greatly different environment and light-matter interaction conditions.

cond-mat.mtrl-sci↗

arXiv2016

Optically induced dynamic nuclear spin polarisation in diamond

The sensitivity of Magnetic Resonance Imaging (MRI) depends strongly on nuclear spin polarisation and, motivated by this observation, dynamical nuclear spin polarisation has recently been applied to enhance MRI protocols (Kurhanewicz, J., et al., Neoplasia 13, 81 (2011)). Nuclear spins associated with the 13 C carbon isotope (nuclear spin I = 1/2) in diamond possess uniquely long spin lattice relaxation times (Reynhardt, E.C. and G.L. High, Prog. in Nuc. Mag. Res. Sp. 38, 37 (2011)) If they are present in diamond nanocrystals, especially when strongly polarised, they form a promising contrast agent for MRI. Current schemes for achieving nuclear polarisation, however, require cryogenic temperatures. Here we demonstrate an efficient scheme that realises optically induced 13 C nuclear spin hyperpolarisation in diamond at room temperature and low ambient magnetic field. Optical pumping of a Nitrogen-Vacancy (NV) centre creates a continuously renewable electron spin polarisation which can be transferred to surrounding 13 C nuclear spins. Importantly for future applications we also realise polarisation protocols that are robust against an unknown misalignment between magnetic field and crystal axis.

quant-ph↗

arXiv2015

Resonance-inclined optical nuclear spin polarization of liquids in diamond structures

Dynamic nuclear polarization (DNP) of molecules in a solution at room temperature has potential to revolutionize nuclear magnetic resonance spectroscopy and imaging. The prevalent methods for achieving DNP in solutions are typically most effective in the regime of small interaction correlation times between the electron and nuclear spins, limiting the size of accessible molecules. To solve this limitation, we design a mechanism for DNP in the liquid phase that is applicable for large interaction correlation times. Importantly, while this mechanism makes use of a resonance condition similar to solid-state DNP, the polarization transfer is robust to a relatively large detuning from the resonance due to molecular motion. We combine this scheme with optically polarized nitrogen vacancy (NV) center spins in nanodiamonds to design a setup that employs optical pumping and is therefore not limited by room temperature electron thermal polarisation. We illustrate numerically the effectiveness of the model in a flow cell containing nanodiamonds immobilized in a hydrogel, polarising flowing water molecules 4700-fold above thermal polarisation in a magnetic field of 0.35 T, in volumes detectable by current NMR scanners.

physics.chem-ph↗