Highly Sensitive MoS2 Photodetectors Enabled with a Dry-Transferred Transparent Carbon Nanotube Electrode.

Fabricating electronic and optoelectronic devices by transferring pre-deposited metal electrodes has attracted considerable attention, owing to the improved device performance. However, the pre-deposited metal electrode typically involves complex fabrication procedures. Here, we introduce our facile electrode fabrication process which is free of lithography, lift-off, and reactive ion etching by directly press-transferring a single-walled carbon nanotube (SWCNT) film. We fabricated Schottky diodes for photodetector applications using dry-transferred SWCNT films as the transparent electrode to increase light absorption in photoactive MoS2 channels. The MoS2 flake vertically stacked with an SWCNT electrode can exhibit excellent photodetection performance with a responsivity of ∼2.01 × 103 A/W and a detectivity of ∼3.2 × 1012 Jones. Additionally, we carried out temperature-dependent current-voltage measurement and Fowler-Nordheim (FN) plot analysis to explore the dominant charge transport mechanism. The enhanced photodetection in the vertical configuration is found to be attributed to the FN tunneling and internal photoemission of charge carriers excited from indium tin oxide across the MoS2 layer. Our study provides a novel concept of using a photoactive MoS2 layer as a tunneling layer itself with a dry-transferred transparent SWCNT electrode for high-performance and energy-efficient optoelectronic devices.

Ventilation and thermal conditions in secondary schools in the Netherlands: Effects of COVID-19 pandemic control and prevention measures.

During the COVID-19 pandemic, the importance of ventilation was widely stressed and new protocols of ventilation were implemented in school buildings worldwide. In the Netherlands, schools were recommended to keep the windows and doors open, and after a national lockdown more stringent measures such as reduction of occupancy were introduced. In this study, the actual effects of such measures on ventilation and thermal conditions were investigated in 31 classrooms of 11 Dutch secondary schools, by monitoring the indoor and outdoor CO2 concentration and air temperature, both before and after the lockdown. Ventilation rates were calculated using the steady-state method. Pre-lockdown, with an average occupancy of 17 students, in 42% of the classrooms the CO2 concentration exceeded the upper limit of the Dutch national guidelines (800 ppm above outdoors), while 13% had a ventilation rate per person (VRp) lower than the minimum requirement (6 l/s/p). Post-lockdown, the indoor CO2 concentration decreased significantly while for ventilation rates significant increase was only found in VRp, mainly caused by the decrease in occupancy (average 10 students). The total ventilation rate per classrooms, mainly induced by opening windows and doors, did not change significantly. Meanwhile, according to the Dutch national guidelines, thermal conditions in the classrooms were not satisfying, both pre- and post-lockdown. While opening windows and doors cannot achieve the required indoor environmental quality at all times, reducing occupancy might not be feasible for immediate implementation. Hence, more controllable and flexible ways for improving indoor air quality and thermal comfort in classrooms are needed.

Solution processed, vertically stacked hetero-structured diodes based on liquid-exfoliated WS2 nanosheets: from electrode-limited to bulk-limited behavior.

Vertically stacked metal-semiconductor-metal heterostructures, based on liquid-processed nanomaterials, hold great potential for various printed electronic applications. Here we describe the fabrication of such devices by spray-coating semiconducting tungsten disulfide (WS2) nanosheets onto indium tin oxide (ITO) bottom electrodes, followed by spraying single-walled carbon nanotubes (SWNTs) as the top electrode. Depending on the formulation of the SWNTs ink, we could fabricate either Ohmic or Schottky contacts at the WS2/SWNTs interface. Using isopropanol-dispersed SWNTs led to Ohmic contacts and bulk-limited devices, characterized by out-of-plane conductivities of ∼10-4 S m-1. However, when aqueous SWNTs inks were used, rectification was observed, due to the formation of a doping-induced Schottky barrier at the WS2/SWNTs interface. For thin WS2 layers, such devices were characterized by a barrier height of ∼0.56 eV. However, increasing the WS2 film thickness led to increased series resistance, leading to a change-over from electrode-limited to bulk-limited behavior at a transition thickness of ∼2.6 µm. This work demonstrates that Ohmic/Schottky behavior is tunable and lays the foundation for fabricating large-area 2D nanosheet-based solution-deposited devices and stacks.

Colors of Single-Wall Carbon Nanotubes.

Although single-wall carbon nanotubes (SWCNTs) exhibit various colors in suspension, directly synthesized SWCNT films usually appear black. Recently, a unique one-step method for directly fabricating green and brown films has been developed. Such remarkable progress, however, has brought up several new questions. The coloration mechanism, potentially achievable colors, and color controllability of SWCNTs are unknown. Here, a quantitative model is reported that can predict the specific colors of SWCNT films and unambiguously identify the coloration mechanism. Using this model, colors of 466 different SWCNT species are calculated, which reveals a broad spectrum of potentially achievable colors of SWCNTs. The calculated colors are in excellent agreement with existing experimental data. Furthermore, the theory predicts the existence of many brilliantly colored SWCNT films, which are experimentally expected. This study shows that SWCNTs as a form of pure carbon, can display a full spectrum of vivid colors, which is expected to complement the general understanding of carbon materials.

Scalable growth of single-walled carbon nanotubes with a highly uniform structure.

Here, a scalable floating catalyst chemical vapor deposition (FCCVD) method is developed for the production of single-walled carbon nanotubes (SWCNTs) with a controlled structure. For the first time, water is used as the growth promoter in the FCCVD process to modulate the growth of SWCNTs. At an optimum water concentration of ca. 115 ppm, the water-assisted FCCVD process synthesizes SWCNTs with a significantly narrow chirality distribution. In particular, the proportion of (9,8) and (8,7) semiconducting tubes was dramatically enhanced to 45% with 27% of the (9,8) tube in the end product. This is attributed to the changes in both the SWCNT diameter and the chiral angle. The experiment results and accurate quantum chemical molecular dynamics simulations show that the addition of water affects the nucleation and the size distribution of nanoparticle catalysts, thus resulting in the growth of SWCNTs with a highly uniform structure. This direct and continuous water-assisted FCCVD provides the possibility for the mass production of high-quality SWCNTs with a controlled structure.

Silicon Substitution in Nanotubes and Graphene via Intermittent Vacancies.

The chemical and electrical properties of single-walled carbon nanotubes (SWCNTs) and graphene can be modified by the presence of covalently bound impurities. Although this can be achieved by introducing chemical additives during synthesis, it often hinders growth and leads to limited crystallite size and quality. Here, through the simultaneous formation of vacancies with low-energy argon plasma and the thermal activation of adatom diffusion by laser irradiation, silicon impurities are incorporated into the lattice of both materials. After an exposure of ∼1 ion/nm2, we find Si-substitution densities of 0.15 nm-2 in graphene and 0.05 nm-2 in nanotubes, as revealed by atomically resolved scanning transmission electron microscopy. In good agreement with predictions of Ar irradiation effects in SWCNTs, we find Si incorporated in both mono- and divacancies, with ∼2/3 being of the first type. Controlled inclusion of impurities in the quasi-1D and -2D carbon lattices may prove useful for applications such as gas sensing, and a similar approach might also be used to substitute other elements with migration barriers lower than that of carbon.

High-performance single-walled carbon nanotube transparent conducting film fabricated by using low feeding rate of ethanol solution.

We report floating catalyst chemical vapour deposition synthesis of single-walled carbon nanotubes (SWCNTs) for high-performance transparent conducting films (TCFs) using low feeding rate of precursor solution. Herein, ethanol acts as carbon source, ferrocene and thiophene as catalyst precursor and growth promoter, respectively. By adopting a low feeding rate of 4 µl min-1, the fabricated TCFs present one of the lowest sheet resistances of ca 78 Ω sq.-1. at 90% transmittance. Optical characterizations demonstrate that the mean diameter of high-quality SWCNTs is up to 2 nm. Additionally, electron microcopy observations provide evidence that the mean length of SWCNT bundles is as long as 28.4 µm while the mean bundle diameter is only 5.3 nm. Moreover, very few CNT loops can be found in the film. Remarkably, the fraction of individual SWCNTs reaches 24.6%. All those morphology data account for the superior optoelectronic performance of our SWCNT TCFs.

Floating catalyst CVD synthesis of single walled carbon nanotubes from ethylene for high performance transparent electrodes.

We have developed the floating catalyst chemical vapor deposition (FCCVD) synthesis of single walled carbon nanotubes (SWCNTs) using C2H4 hydrocarbon as a carbon source and iron nanoparticles as the catalyst in an environmentally friendly and economical process. For the first time, ethylene was used as the only carbon source in FCCVD with N2 as the main carrier gas. No sulphur and less than 15% H2 in a N2 carrier gas were used. By varying the ferrocene concentration, the diameter of the SWCNTs was tuned in the range of 1.3-1.5 nm with the optimized control of ferrocene concentration. The process produced SWCNTs with an average length of 13 µm and with a low level of bundling, that is a high proportion (28%) of individual tubes. The electron diffraction (ED) pattern indicated a random chirality distribution of the tubes between armchair and zigzag structures. The ED analysis also revealed that 35-38% of tubes are metallic. As a result of having long SWCNTs with a low level of bundling and a high fraction of metallic tubes, we produced a highly conductive transparent film with a sheet resistance of 51 Ohm per sq. for 90% transmission at 550 nm after HNO3 treatment, this being one of the lowest sheet resistance values reported for SWCNT thin films.

Wafer-Scale Thermophoretic Dry Deposition of Single-Walled Carbon Nanotube Thin Films.

We report the direct and dry deposition of transparent conducting films (TCFs) of aerosol-synthesized single-walled carbon nanotubes (SWNTs) using a thermophoretic precipitator (TP) designed for the uniform and efficient deposition of aerosol-synthesized nanomaterials on 50 mm wafers or similarly sized polymer substrates. The optical and electrical performance of the fabricated TCFs match or surpass the published results achieved using a filter-based collection of aerosol-synthesized SWNTs, and TCFs with sheet resistances of 60 Ω/sq. at 87.8% transmittance and 199 Ω/sq. at 96% transmittance on flexible polymer substrates are demonstrated. The precipitator design is immediately applicable in roll-to-roll fabrication of SWNT TCFs or other functional coatings of aerosol-synthesized nanomaterials.

Highly conductive and transparent single-walled carbon nanotube thin films from ethanol by floating catalyst chemical vapor deposition.

Single-walled carbon nanotube (SWCNT) films have great potential to replace indium tin oxide films for applications in transparent and conductive electronics. Here we report a high yield production of SWCNT transparent conducting films (TCFs) by the floating catalyst chemical vapor deposition method using ethanol as the carbon source. To the best of our knowledge, this is the first report regarding SWCNT TCFs using ethanol as the carbon source. The fabricated uniform SWCNT TCFs exhibit a competitive sheet resistance of 95 Ω sq-1 at 90% transmittance after doping with AuCl3. The SWCNT TCFs possess high quality and the mean length of SWCNT bundles is approximately 27.4 µm. Furthermore, the concentration of semiconducting SWCNTs is 75-77%. Additionally, the chirality maps obtained from electron diffraction analysis demonstrate that our SWCNTs are biased towards the armchair type.

Hierarchical chrysanthemum-flower-like carbon nanomaterials grown by chemical vapor deposition.

Novel hierarchical chrysanthemum-flower-like carbon nanomaterials (CFL-CNMs) were synthesized by thermal chemical vapor deposition based on acetylene decomposition. A scanning electron microscope and a transmission electron microscope were employed to observe the morphology and structure of the unconventional nanostructures. It is found that the CFL-CNMs look like a blooming chrysanthemum with a stem rather than a spherical flower. The carbon flower has an average diameter of 5 µm, an average stem diameter of 150 nm, branch diameters ranging from 20 to 70 nm, and branch lengths ranging from 0.5 to 3 µm. The morphologies of the CFL-CNMs are unlike any of those previously reported. Fishbone-like carbon nanofibers with a spindle-shaped catalyst locating at the tip can also be found. Furthermore, the catalyst split was proposed to elucidate the formation mechanism of CFL-CNMs. A large and glomerate catalyst particle at the tip of the carbon nanofiber splits into smaller catalyst particles which are catalytic-active points for branch formation, resulting in the formation of CFL-CNMs.

A timesaving, low-cost, high-yield method for the synthesis of ultrasmall uniform graphene oxide nanosheets and their application in surfactants.

Graphene oxide nanosheets (GONSs) with a lateral size less than 100 nm have attracted more and more attention for their wide range of potential applications, from bionanotechnology and nanobiomedicine to surfactants. However, at present GONSs are commonly prepared from graphite nanofibers or graphite nanopowders which are both expensive. Here, a timesaving, low-cost, high-yield method is proposed for preparing ultrasmall uniform GONSs with an average lateral size of ∼30 nm, utilizing common graphite powder as the raw material in the absence of a strong acid. The obtained GONSs are able to disperse single-walled carbon nanotubes (SWCNTs) effectively, and the dispersion could withstand high-speed centrifugation. Consequently, GONSs could indeed serve as a superior surfactant for the dispersion of SWCNTs, and the dispersion could be further applied in electronics, as the GONSs may be further reduced to reduced GONSs or graphene nanosheets.

Y-junction carbon nanocoils: synthesis by chemical vapor deposition and formation mechanism.

Y-junction carbon nanocoils (Y-CNCs) were synthesized by thermal chemical vapor deposition using Ni catalyst prepared by spray-coating method. According to the emerging morphologies of Y-CNCs, several growth models were advanced to elucidate their formation mechanisms. Regarding the Y-CNCs without metal catalyst in the Y-junctions, fusing of contiguous CNCs and a tip-growth mechanism are considered to be responsible for their formation. However, as for the Y-CNCs with catalyst presence in the Y-junctions, the formation can be ascribed to nanoscale soldering/welding and bottom-growth mechanism. It is found that increasing spray-coating time for catalyst preparation generates agglomerated larger nanoparticles strongly adhering to the substrate, resulting in bottom-growth of CNCs and appearance of the metal catalyst in the Y-junctions. In the contrary case, CNCs catalyzed by isolated smaller nanoparticles develop Y-junctions with an absence of metal catalyst by virtue of weaker adhesion of catalyst with the substrate and tip-growth of CNCs.

How is the AIE mechanism profoundly changed in an ESIPT family: the novel introduction of a tetraphenylethene group onto (Z)-3-(quinolin-2-ylmethylene)-3,4-dihydroquinoxalin-2(1H)-one.

It is reported that two derivatives of (Z)-3-(quinolin-2-ylmethylene)-3,4-dihydroquinoxalin-2-(1H)-one (1) with a tetraphenylethene (TPE) group introduced at amide N atom of the dihydroquinoxalinone moiety (2) or at phenyl ring of the quinoline fragment (3) are synthesized, and the derivatives exhibit a remarkably enhanced aggregation-induced emission (AIE) activity than the parent. Although both the parent and the derivatives have the characteristic of an excited state intramolecular proton transfer (ESIPT), the AIE mechanism of 2 and 3 is totally different from that of 1. The considerably stronger emission of 3 than that of 2 should be attributed to the unique crystallization-induced emission enhancement (CIEE) effect.

Risk factors on human cystic echinococcosis in Hobukesar Mongolian Autonomous County in Xinjiang / 中华流行病学杂志

Objective To study the risk factors of human cystic echinococcosis (CE) in Hobukesar Mongolian Autonomous County of Xinjiang (HMACX) and to discuss the related strategies for prevention and control. Methods A randomized sampling method was used to screen local residents for human CE in HMACX. CE related risk factors including ethnicity,age,sex,occupation and personal status on hygiene etc. were analyzed under multi-factor logistic regression. Results The prevalence rates of CE and alveolar echinococcosis (AE) were 3.7% (23/627) and 0.16%(1/627) respectively,with the seropositive rate as 12.4% (76/613). The main risk factors that significantly associated with CE were age (OR=7.6,95% CI: 2.481-23.579) and slaughtering livestock in the households (OR=3.2,95%CI: 1.297-7.809). Herdsmen had the highest prevalence of CE in all of the occupations in this study. Conclusion HMACX had been a highly endemic area for human CE,with age and family slaughtering-livestock-behavior appeared to be the main possible risk factors.

Fas ligand expression in colon cancer: a possible mechanism of tumor immune privilege.

AIM: To detect the expression of Fas ligand (FasL) in colon cancer tissues and cell lines and analyze the function of FasL-expressing colon cancer cells in inducing Fas-sensitive T lymphocyte apoptosis. METHODS: Ninety surgically resected colon cancer tissues and 15 hepatic metastasis specimens were investigated by immunohistochemical method with normal colon mucosa and colon adenoma as control. The relationship between FasL expression and pathologic features was also analyzed. FasL expression of 4 colon cancer cell lines, SW620, Lovo, LS-174T and SW1116, were detected by Western blotting assay. The function of FasL expressed on colon cancer cells was determined by coculture assay with Jurkat T lymphocytes, the apoptotic rate of which was detected by flow cytometry assay. RESULTS: Fifty-six (62.22%) cases of all the 90 colon cancer tissues and all (100%) the liver metastasis specimens expressed FasL, significantly higher than normal colon mucosa and colonic adenoma. Higher expression of FasL was found in more advanced stage of colon cancer and in cancer tissues with lymphatic or hepatic metastasis. All the colon cancer cell lines were found to express FasL. After coculture with the SW1116 cells for 24 h with an effector: target ratio 10:1, the rate of apoptosis of Jurkat cells rose from 1.9% to 21.0%. CONCLUSION: The expression of FasL is upregulated in colon cancer and the functionally expressed FasL can induce apoptosis of Fas-expressing T lymphocytes.

[A comparative study on burn wound healing treated by different methods of recombinant human epidermal growth factor].

OBJECTIVE: To explore an optimal method of recombinant human epidermal growth factor(rhEGF) application on the burn wounds of superficial II degree and profound II degree for accelerating its healing. METHODS: There were 180 burn wounds in 60 patients with the self-corresponding wound of the same degree as controls. The wounds of all patients were divided three regions(A, B, C). The wounds were treated once a day with 1% SD-Ag in region A as controls, with rhEGF(40 U/cm2) in region B, and with a combination of rhEGF(40 U/cm2) and Su Yu Ping (5 g) in region C. The wound healing time was recorded and compared. RESULTS: In regions A, B and C, the healing time of superficial II degree wound was (13.20 +/- 2.40) days, (10.20 +/- 2.20) days and (8.72 +/- 2.31) days (P < 0.01); that of profound II degree wound was (20.10 +/- 3.40) days, (17.20 +/- 3.12) days and (15.10 +/- 3.81) days respectively (P < 0.01, P < 0.05). The healed wound of profound II degree was elastic and tough in regions B and C, while that was not elastic and tough, and congestive in region A. CONCLUSION: The above results indicate that rhEGF can enhance burn wound healing markedly and that a combination rhEGF and Su Yu Ping has more significant effect than rhEGF alone and is recommended for clinical application.

Structure and Function of a Novel Silencer in 5' Flanking Distal Region of Human beta Globin Gene.

A novel silencer fragment(310 bp) was recently discovered and identified to locate between -2 132 bp and -1 822 bp upstream from the cap site of beta-globin gene by gel retardation assay and luciferase reporter gene expression analysis. DNA footprinting assays were performed to determine the interaction between its DNA sequence and binding proteins from the nuclear extract of Hela cells. The results showed that there were two nuclear protein binding sites in thissilencer, one was the -2 017--2 011 bp sequence CTTCCGC" and the other was the -2 006--1 997 bp sequence "CACTTTATTT". Two sequences were mutated into "CTTAAGC" and "CACTTAAGTT", respectively by two mutagenic primer pairs, in order to construct two mutation types of the 310 bp fragment. The competitive gel retardation assays showed that two mutation types of the 310 bp fragment and their four smaller DNA fragments, which were formed respectively by the digestion of restriction enzyme BspTI, all lost their competitive ability against the wild type of 310 bp fragment probe for DNA-binding proteins without exception. Furthermore, the double-strand oligonucleotides, which contained both the sequences of "CTTCCGC" and "CACTTTATTT", were synthesized, and the competitive gel retardation assays showed that they competed ability against wild type 310 bp fragment probe for DNA-binding proteins. The results suggest that two binding sites of the nuclear proteins are involved or associated with a potential DNA-DNA interaction. Moreover, the specific DNA-binding proteins were purified from the nuclear extract of Hela cells by using "DNA-binding protein purification kit" for magnetic isolation. In order to identify the purified DNA-binding proteins, a SDS-PAGE was performed. By using the silver staining, the PAGE electrophoretogram showed that these two nuclear proteins specifically bound to these two sites of the silencer, appearing as two definite bands. The molecular weight of each protein was determined to be 37 kD or 81 kD, respectively.

Function and Identification of a Novel Silencer in 5' Flanking Distal Region of Human beta Globin Gene.

In order to inquire into the possible function of the sequence in the 5' flanking distal region and/or delta-beta intergene of human beta-globin gene several recombinant retrovirus vectors containing beta-globin gene with different length of 5' flanking sequences were constructed and transfected into Hela cells. The levels of beta-globin gene expression determined quantitatively by Northern blotting and RT-cPCR revealed that there is a strong silencer activity in a DNA sequence(723 bp) between-2282 bp and -1559 bp upstream from the cap site of beta-globin gene. The silencer activity was further identified to be possibly located mainly on a 310bp DNA restriction fragment by using the competitive gel retardation assay(cGRA) and many known regulatory elements were found present in it as well. Luciferase reporter gene expression system was adopted to study the function of this 310bp fragment and data showed that the fragment exhibited significant inhibitory effect on each of the recombinant vectors with alpha,beta or delta gene promoters. The above results suggest that this silencer may be involved in a potential regulatory network between delta and beta globin genes and may play an important role in the regulation of globin gene expression.