Arson presents a challenging crime scene for fire investigators worldwide. Key to the investigation of suspected arson cases is the analysis of fire debris for the presence of accelerants or ignitable liquids. This st...Arson presents a challenging crime scene for fire investigators worldwide. Key to the investigation of suspected arson cases is the analysis of fire debris for the presence of accelerants or ignitable liquids. This study has investigated the application and method development of vapor phase mid-Infrared (mid-IR) spectroscopy using a field portable quantum cascade laser (QCL) based system for the detection and identification of accelerant residues such as gasoline, diesel, and ethanol in fire debris. A searchable spectral library of various ignitable fluids and fuel components measured in the vapor phase was constructed that allowed for real-time identification of accelerants present in samples using software developed in-house. Measurement of vapors collected from paper material that had been doused with an accelerant followed by controlled burning and then extinguished with water showed that positive identification could be achieved for gasoline, diesel, and ethanol. This vapor phase mid-IR QCL method is rapid, easy to use, and has the sensitivity and discrimination capability that make it well suited for non-destructive crime scene sample analysis. Sampling and measurement can be performed in minutes with this 7.5 kg instrument. This vibrational spectroscopic method required no time-consuming sample pretreatment or complicated solvent extraction procedure. The results of this initial feasibility study demonstrate that this portable fire debris analyzer would greatly benefit arson investigators performing analysis on-site.展开更多
Bilayer(BL)transition metal dichalcogenides(TMDs)are important materials in valleytronics and twistronics.Here we study terahertz(THz)magneto-optical(MO)properties of n-type 2H-stacking BL molybdenum sulfide(MoS_(2))o...Bilayer(BL)transition metal dichalcogenides(TMDs)are important materials in valleytronics and twistronics.Here we study terahertz(THz)magneto-optical(MO)properties of n-type 2H-stacking BL molybdenum sulfide(MoS_(2))on sapphire substrate grown by chemical vapor deposition.The AFM,Raman spectroscopy and photoluminescence are used for characterization of the samples.Applying THz time-domain spectroscopy(TDS),in combination with polarization test and the presence of magnetic field in Faraday geometry,THz MO transmissions through the sample are measured from O to 8 T at 80 K.The complex right-and left-handed circular MO conductivities for 2H-stacking BL MoS_(2) are obtained.Through fitting the experimental results with theoretical formula of MO conductivities for an electron gas,generalized by us previously through the inclusion of photon-induced electronic backscat-tering effect,we are able to determine magneto-optically the key electronic parameters of BL MoS_(2),such as the electron density ne,the electronic relax-ation time T,the electronic localization factor c and,particularly,the effective electron mass m*around Q-point in between the K-and I-point in the elec-tronic band structure.The dependence of these parameters upon magnetic field is examined and analyzed.This is a pioneering experimental work to measure m*around the Q-point in 2H-stacking BL MoS_(2) and the experimental value is very close to that obtained theoretically.We find that ne/τ/|c|/m^(*)in 2H-stacking BL MoS_(2) decreases/increases/decreases/increases with increasing magnetic field.The results obtained from this study can be benefit to us in gaining an in-depth understanding of the electronic and optoelectronic properties of BL TMD systems.展开更多
Bilayer (BL) molybdenum disulfide (MoS_(2)) is one of the most important electronic structures not only in valleytronics but also in realizing twistronic systems on the basis of the topological mosaics in moiré s...Bilayer (BL) molybdenum disulfide (MoS_(2)) is one of the most important electronic structures not only in valleytronics but also in realizing twistronic systems on the basis of the topological mosaics in moiré superlattices. In this work, BL MoS_(2) on sapphire substrate with 2H-stacking structure is fabricated. We apply the terahertz (THz) time-domain spectroscopy (TDS) for examining the basic optoelectronic properties of this kind of BL MoS_(2). The optical conductivity of BL MoS_(2) is obtained in temperature regime from 80 K to 280 K. Through fitting the experimental data with the theoretical formula, the key sample parameters of BL MoS_(2) can be determined, such as the electron density, the electronic relaxation time and the electronic localization factor. The temperature dependence of these parameters is examined and analyzed. We find that, similar to monolayer (ML) MoS_(2), BL MoS_(2) with 2H-stacking can respond strongly to THz radiation field and show semiconductor-like optoelectronic features. The theoretical calculations using density functional theory (DFT) can help us to further understand why the THz optoelectronic properties of BL MoS_(2) differ from those observed for ML MoS_(2). The results obtained from this study indicate that the THz TDS can be applied suitably to study the optoelectronic properties of BL MoS_(2) based twistronic systems for novel applications as optical and optoelectronic materials and devices.展开更多
文摘Arson presents a challenging crime scene for fire investigators worldwide. Key to the investigation of suspected arson cases is the analysis of fire debris for the presence of accelerants or ignitable liquids. This study has investigated the application and method development of vapor phase mid-Infrared (mid-IR) spectroscopy using a field portable quantum cascade laser (QCL) based system for the detection and identification of accelerant residues such as gasoline, diesel, and ethanol in fire debris. A searchable spectral library of various ignitable fluids and fuel components measured in the vapor phase was constructed that allowed for real-time identification of accelerants present in samples using software developed in-house. Measurement of vapors collected from paper material that had been doused with an accelerant followed by controlled burning and then extinguished with water showed that positive identification could be achieved for gasoline, diesel, and ethanol. This vapor phase mid-IR QCL method is rapid, easy to use, and has the sensitivity and discrimination capability that make it well suited for non-destructive crime scene sample analysis. Sampling and measurement can be performed in minutes with this 7.5 kg instrument. This vibrational spectroscopic method required no time-consuming sample pretreatment or complicated solvent extraction procedure. The results of this initial feasibility study demonstrate that this portable fire debris analyzer would greatly benefit arson investigators performing analysis on-site.
基金supported by the National Natural Science Foundation of China(NSFC)(Grant Nos.U2230122 and U2067207)Shenzhen Science and Technology Program(No.KQTD20190929173954826).
文摘Bilayer(BL)transition metal dichalcogenides(TMDs)are important materials in valleytronics and twistronics.Here we study terahertz(THz)magneto-optical(MO)properties of n-type 2H-stacking BL molybdenum sulfide(MoS_(2))on sapphire substrate grown by chemical vapor deposition.The AFM,Raman spectroscopy and photoluminescence are used for characterization of the samples.Applying THz time-domain spectroscopy(TDS),in combination with polarization test and the presence of magnetic field in Faraday geometry,THz MO transmissions through the sample are measured from O to 8 T at 80 K.The complex right-and left-handed circular MO conductivities for 2H-stacking BL MoS_(2) are obtained.Through fitting the experimental results with theoretical formula of MO conductivities for an electron gas,generalized by us previously through the inclusion of photon-induced electronic backscat-tering effect,we are able to determine magneto-optically the key electronic parameters of BL MoS_(2),such as the electron density ne,the electronic relax-ation time T,the electronic localization factor c and,particularly,the effective electron mass m*around Q-point in between the K-and I-point in the elec-tronic band structure.The dependence of these parameters upon magnetic field is examined and analyzed.This is a pioneering experimental work to measure m*around the Q-point in 2H-stacking BL MoS_(2) and the experimental value is very close to that obtained theoretically.We find that ne/τ/|c|/m^(*)in 2H-stacking BL MoS_(2) decreases/increases/decreases/increases with increasing magnetic field.The results obtained from this study can be benefit to us in gaining an in-depth understanding of the electronic and optoelectronic properties of BL TMD systems.
基金supported by the National Natural Science Foundation of China(NSFC)(Grant Nos.U2230122 and U2067207)Shenzhen Science and Technology Program(No.KQTD20190929173954826).
文摘Bilayer (BL) molybdenum disulfide (MoS_(2)) is one of the most important electronic structures not only in valleytronics but also in realizing twistronic systems on the basis of the topological mosaics in moiré superlattices. In this work, BL MoS_(2) on sapphire substrate with 2H-stacking structure is fabricated. We apply the terahertz (THz) time-domain spectroscopy (TDS) for examining the basic optoelectronic properties of this kind of BL MoS_(2). The optical conductivity of BL MoS_(2) is obtained in temperature regime from 80 K to 280 K. Through fitting the experimental data with the theoretical formula, the key sample parameters of BL MoS_(2) can be determined, such as the electron density, the electronic relaxation time and the electronic localization factor. The temperature dependence of these parameters is examined and analyzed. We find that, similar to monolayer (ML) MoS_(2), BL MoS_(2) with 2H-stacking can respond strongly to THz radiation field and show semiconductor-like optoelectronic features. The theoretical calculations using density functional theory (DFT) can help us to further understand why the THz optoelectronic properties of BL MoS_(2) differ from those observed for ML MoS_(2). The results obtained from this study indicate that the THz TDS can be applied suitably to study the optoelectronic properties of BL MoS_(2) based twistronic systems for novel applications as optical and optoelectronic materials and devices.
