Optical contrast determination of the thickness of SiO_2 film on Si substrate partially covered by two-dimensional crystal flakes

SiO_2/Si substrate has been widely used to support two-dimensional （2-D） crystal flakes grown by chemical vapor deposition or prepared by micromechanical cleavage. The visibility of 2-D flakes is very sensitive to the thickness of the SiO_2 layer （hsiO_2）, which can not be determined precisely after the deposit of 2-D flakes. Here, we demonstrated a simple, fast and nondestructive tech- nique to precisely determine hsiO_2 of SiO_2 films on Si substrate only by optical contrast measurement with a typical micro-Raman confocal system. Because of its small lateral resolution down to the micrometer scale, this tech- nique can be used to access hsiO_2 on SiO_2/Si substrate that has been partially covered by 2-D crystal flakes, and then further determine the layer number of the 2-D crystal flakes. This technique can be extended to other dielectric multilayer substrates and the layer-number determination of 2-D crystal flakes on those substrates.

A Cathodic Electrochromic Material Based on Thick Perylene Bisimide Film with High Optical Contrast and High Stability

Cathodic electrochromic materials realized by n-type doping of conducting polymers are scarce.Even with limited cases reported in the literature,long-term stability is an urgent problem to be solved.Herein,we report a high performance and stable cathodic electrochromic material,poly(Th-Cl-PBI),based on a perylene bisimide function core.Due to its high electropolymerization efficiency and low steric hindrance of thiophene groups,perylene bisimide,which generally does not dissolve and form film easily,can grow to the sufficient thickness of 350 nm for wide-range spectral modulation and large-capacity energy storage.The optical contrast of optimized film with a thickness of 240 nm reaches as high as 69.1% at 520 nm,94.1% at 760 nm,and 95.7% at 680 nm.Theoretical simulation based on the Lambert–Beer law also verifies this optical contrast dependent on film thickness.Besides,during the transition between the neutral state and radical anion state,it maintains 90.2% of the electrochemical activity after 4000 cycles,and the transmittance spectrum remains stable after 2000 cycles.The cation size effect on cathodic electrochromic process and cyclic stability has been investigated.

Development of a handheld diffuse optical breast cancer assessment probe

Diffuse Optical Spectroscopy(DOS)is a promising non-invasive and non-ionizing technique for breast anomaly detection.In this study,we have developed a new handheld DOS probe to measure optical properties of breast tissue.In the proposed probe,the breast tissue is illuminated with four near infrared(NIR)wavelengths light emitting diodes(LED),which are encapsulated in a package(eLEDs),and two PIN photodiodes measure the intensity of the scattered photons at two di®erent locations.The proposed technique of using eLEDs is introduced,in order to have a multi-wavelength pointed-beam illumination source instead of using the laser-coupledflberoptic technique,which increases the complexity,size,and cost of the probe.Despite the fact that the proposed technique miniaturizes the probe and reduces the complexity of the DOS,the study proves that it is accurate and reliable in measuring optical properties of the tissue.The measurements are performed at the rate of 10 Hz which is suitable for dynamic measurement of biological activity,in-vivo.The multi-spectral evaluation algorithm is used to reconstruct four main absorber concentrations in the breast including oxy-hemoglobin(cHb),deoxy-hemoglobin(cHbO_(2)),water(cH2O),fat(cFat),and average scattering coeficient of the medium,as well as concentration changes in Hb(
cHb)and HbO_(2)(
cHbO_(2)).Although the probe is designed for breast cancer diagnosis,it can be used in a wide range of applications for both static and dynamic measurements such as functional brain imaging.A series of phantoms,comprised of Delrinr,Intralipidr,PierceTM and Black ink,are used to verify performance of the device.The probe will be tested on human subjects,in-vivo,in the next phase.

Hybrid Electrochromic Multilayer Films Based on Fe(II)-Metalloviologens and TiO_2 Nanoparticles

Metallosupramolecular coordination polyelectrolyte, Fe（II）-metalloviologen（FEN）, was prepared by the reaction of Fe（II） with a novel bisterpyridine ligand. As active components, FENs could be assembled into electrochromic multilayer films with negative charged polystyrene sulfate（PSS） by the sequential deposition layer-by-layer technique. Numerous analytical instruments, such as UV-Vis spectroscopy, atomic force microscopy（AFM）, tunneling electron microscopy（TEM）, zeta-potential measurement and electrochemical measurement have been utilized to characterize their morphology, optical and electrochromic properties. It has been observed that as-prepared films exhibited multi-colour changes by triggering with different potentials. However, the low optical contrast of multilayer films would limit their further applications. In order to overcome this problem, semiconductor TiO2 nanoparticles（TiO2） were incorporated into FEN multilayers by layer-by-layer approach. By carefully optimizing the film structure, as-resulted hybrid films containing FEN, TiO2 and PSS exhibited high optical contrast, suitable response time and long-term stability. Such hybrid films should be promising candidates to meet the requirements for developing flexible displays and electrochromic devices.

Investigation of multilayer domains in large-scale CVD monolayer graphene by optical imaging

CVD graphene is a promising candidate for optoelectronic applications due to its high quality and high yield.However,multi-layer domains could inevitably form at the nucleation centers during the growth.Here,we propose an optical imaging technique to precisely identify the multilayer domains and also the ratio of their coverage in large-scale CVD monolayer graphene.We have also shown that the stacking disorder in twisted bilayer graphene as well as the impurities on the graphene surface could be distinguished by optical imaging.Finally,we investigated the effects of bilayer domains on the optical and electrical properties of CVD graphene,and found that the carrier mobility of CVD graphene is seriously limited by scattering from bilayer domains.Our results could be useful for guiding future optoelectronic applications of large-scale CVD graphene.

Plasma-assisted fabrication of monolayer phosphorene and its Raman characterization

There have been continuous efforts to seek novel functional two-dimensional semiconductors with high performance for future applications in nanoelectronics and optoelectronics. In this work, we introduce a successful experimental approach to fabricate monolayer phosphorene by mechanical cleavage and a subsequent Ar＊ plasma thinning process. The thickness of phosphorene is unambiguously determined by optical contrast spectra combined with atomic force microscopy （AFM）. Raman spectroscopy is used to characterize the pristine and plasma-treated samples. The Raman frequency of the A2g mode stiffens, and the intensity ratio of A2g to Alg modes shows a monotonic discrete increase with the decrease of phosphorene thickness down to a monolayer. All those phenomena can be used to identify the thickness of this novel two-dimensional semiconductor. This work on monolayer phosphorene fabrication and thickness determination will facilitate future research on phosphorene.

Te-seeded growth of few-quintuple layer Bi2Te3 nanoplates

We report on a Te-seeded epitaxial growth of ultrathin Bi2Te3 nanoplates （down to three quintuple layers （QL）） with large planar sizes （up to tens of micrometers） through vapor transport. Optical contrast has been systematically investigated for the as-grown Bi2Te3 nanoplates on the SiO2/Si substrates, experimentally and computationally. The high and distinct optical contrast provides a fast and convenient method for the thickness determination of few-QL Bi2Te3 nanoplates. By aberration-corrected scanning transmission electron microscopy, a hexagonal crystalline structure has been identified for the Te seeds, which form naturally during the growth process and initiate an epitaxial growth of the rhombohedral- structured Bi2Te3 nanoplates. The epitaxial relationship between Te and Bi2T% is identified to be perfect along both in-plane and out-of-plane directions of the layered nanoplate. Similar growth mechanism might be expected for other bismuth chalcogenide layered materials.

Rapid identification of two-dimensional materials via machine learning assisted optic microscopy

A combination of Fresnel law and machine learning method is proposed to identify the layer counts of 2D materials.Three indexes,which are optical contrast,red-green-blue,total color difference,are presented to illustrate and simulate the visibility of 2D materials on Si/SiO_(2) substrate,and the machine learning algorithms,which are k-mean clustering and k-nearest neighbors,are employed to obtain thickness database of 2D material and test the optical images of 2D materials via red-green-blue index.The results show that this method can provide fast,accurate and large-area property of 2D material.With the combination of artificial intelligence and nanoscience,this machine learning assisted method eases the workload and promotes fundamental research of 2D materials.

Rapidly counting atomic planes of ultra-thin MoSe2 nanosheets(1≤n≤4)on SiO2/Si substrate

The optical, thermal and electrical properties of ultra-thin two-dimensional （2D） crystal materials are highly related to their thickness. Therefore, identifying the atomic planes of few-layer crystal materials rapidly is crucial to fundamental study. Here, a simple technique was demonstrated based on optical contrast for counting atomic planes （n） of few-layer MoSe2 on SiO2/Si substrates. It is found that the optical contrast of single-layer MoSe2 depends on light wavelength and thickness of SiO2 on Si substrate. The data calculated based on a Fresnel law-based model as well as atomic force microscopy （AFM） mea- surements fit well with the values measured by spectro- scopic ellipsometer. Furthermore, the calculated and measured contrasts were integral and plotted, which can be used to determine the MoSe2 atomic planes （1 ≤ n ≤ 4） accurately and rapidly.

Self-powered macroscopic Brownian motion of spontaneously running liquid metal motors

We disclosed the interiorly driven macroscopic Brownian motion behavior of self-powered liquid metal motors. Such tiny motors in millimeter scale move randomly at a velocity magnitude of centimeters per second in aqueous alkaline solution, well resembling the classical Brownian motion. However, unlike the existing phenomena, where the particle motions were caused by collisions from the surrounding molecules, the current random liquid metal motions are internally enabled and self-powered, along with the colliding among neighboring motors, the substrate and the surrounding electrolyte molecules. Through uniformly dissolving only 1% （mass percentage） A1 into GaInl0, many tiny motors can be quickly fabricated and activated to take the Brownian-like random motions. Further, we introduced an experimental approach of using optical image contrast, which works just like the Wilson cloud chamber, to distinctively indicate the motor trajectory resulted from the generated hydrogen gas stream. A series of unusual complicated multi-phase fluid mechanics phenomena were observed. It was also identified that the main driving factor of the motors comes from the H2 bubbles generated at the bottom of these tiny motors, which is different from the large size self-fueled liquid metal machine. Several typical mechanisms for such unconventional Brownian-like motion phenomena were preliminarily interpreted.