Semiconductor Gas Sensors: Dry Synthesis and Application

Since the development of the first chemoresistive metal oxide based gas sensors, transducers with innovative properties have been prepared by a variety of wet‐ and dry‐deposition methods. Among these, direct assembly of nanostructured films from the gas phase promises simple fabrication and control and with the appropriate synthesis and deposition methods nm to μm thick films, can be prepared. Dense structures are achieved by tuning chemical or vapor deposition methods whereas particulate films are obtained by deposition of airborne, mono‐ or polydisperse, aggregated or agglomerated nanoparticles. Innovative materials in non‐equilibrium or sub‐stoichiometric states are captured by rapid cooling during their synthesis. This Review presents some of the most common chemical and vapor‐deposition methods for the synthesis of semiconductor metal oxide based detectors for chemical gas sensors. In addition, the synthesis of highly porous films by novel aerosol methods is discussed. A direct comparison of structural and chemical properties with sensing performance is given.     

Polymer assisted deposition

Polymer assisted deposition (PAD) is a chemical solution route to high quality thin films of metal oxides. This technique employs metal ions coordinated to polymers as the film precursor. The use of polymer bound metals has several advantages. The polymer controls the viscosity and binds metal ions, resulting in a homogeneous distribution of metal precursors in the solution and the formation of uniform metal oxide films. The nature of the metal oxide deposition is dominated by bottom-up growth, leading to ready formation of crack-free epitaxial metal oxides and the ability to coat nanofeatured substrates in a conformal fashion.     

A new route to fabricate large-area, compact Ag metal mesh films with ordered pores

Ordered Si nanowire (SiNW) arrays can be fabricated by metal-assisted chemical etching. The metal mesh films (MMFs) are extremely important for achieving a high quality of the SiNWs. We have developed a two-step chemical deposition method to obtain compact porous Ag MMFs. By the separation of the nucleation and growth stages of the metal in the two-step deposition processes, the overgrowth of the metals to form randomly aggregated irregular metal particles can be overcome. Hexagonally arranged polystyrene (PS) latex microspheres have been employed as a template for the deposition of porous Ag MMFs. The spacing of the pores in the Ag MMFs is determined by the diameter of PS microspheres, and the pore size can also be tuned by changing Ar plasma etching time. One of the main advantages of the two-step deposition method lies in that Ag MMFs can be produced with PS microspheres that are not limited to a single layer, which dramatically simplifies the tedious processes of producing a monolayered PS template. The two-step chemical deposition method shows great potential in metal-assisted chemical etching.     

聚合物辅助设计生长高质量多功能薄膜

随着薄膜材料的日益发展和新型薄膜材料的不断涌现,开发薄膜生长技术对于半导体和光电等科技领域的作用日益突出。本文主要介绍最近发展的聚合物辅助沉积从分子层面上控制生长高质量的薄膜材料。聚合物辅助沉积是一种生长高质量薄膜的化学水性溶液方法,将金属离子与聚合物通过络合、氢键或静电等方式形成一种均匀稳定的前驱体溶液,再经过超滤、成膜和热处理形成高质量的金属氧化物、金属碳化物、金属氮化物、金属单质、金属硫/硒化物等薄膜以及纳米粒子等化合物或复合功能材料。该方法中水溶性的聚合物能通过络合作用抑制金属离子的水解使得溶液稳定,并能精确控制薄膜的组分从而形成高质量的薄膜。该化学溶液方法的提出为科学技术领域提供了一种低成本和大面积制备薄膜的技术路线。本文最后总结和展望了聚合物辅助沉积法未来的挑战和发展方向。     

Alloys prepared by plasma-enhanced chemical vapor deposition (PECVD)

Three different techniques for the deposition of thin metal alloy films by plasma-enhanced chemical vapor deposition are described. These are the joint vaporization of a mixture of precursors, the use of separate sources connected directly to the reactor, and finally, the use of several reservoirs arranged in series. Various organometallics have been used as precursors to prepare combinations of Fe/Co and Au/Pt/Pd.     

Study of the first nucleation steps of thin films by XPS inelastic peak shape analysis

The initial steps in the formation of thin films have been investigated by analysis of the peak shape (both inelastic background and elastic contributions) of X‐ray photoelectron spectra. Surface coverage and averaged height of the deposited particles have been estimated for several overlayers (nanometre range) after successive deposition cycles. This study has permitted the assessment of the type of nucleation and growth mechanisms of the films. The experiments have been carried out in situ in the preparation chamber of an XPS spectrometer. To check the performance of the method, several materials (i.e. cerium oxide, vanadium oxide and cadmium sulfide) have been deposited on different substrates using a variety of preparation procedures (i.e. thermal evaporation, ion beam assisted deposition and plasma enhanced chemical vapour deposition). It is shown that the first deposited nuclei of the films are usually formed by three‐dimensional particles whose heights and degree of surface coverage depend on the chemical characteristics of the growing thin film and substrate materials, as well as the deposition procedure. It is concluded that XPS peak shape analysis can be satisfactorily used as a general method to characterize morphologically the first nanometric moieties that nucleate a thin film. Copyright © 2006 John Wiley & Sons, Ltd.     

A secondary ion mass spectrometric study of thallium oxide thin films grown via metal organic chemical vapour deposition

Within a comprehensive programme including synthesis via metal organic chemical vapour deposition (MOCVD) and characterization of inorganic compounds and materials of possible interest in technologies based on thin films, results concerning the deposition of metal oxides by means of volatile organometal precursors are reported. In particular, thallium oxide films obtained by the MOCVD technique and commercial powders of Tl₂O₃ and Tl₂O adsorbed on several metal substrates (stainless steel, Si, Cu, Mo, Pt) were studied by secondary ion mass Spectrometry (SIMS) under ion beam bombardment at different ion energies. The positive- and negative-ion mass spectra exhibit typical isotopic patterns of several ionic species produced by interesting interfacial reactions, and the analysis of their relative abundances provides a measure of oxide reactivity towards different substrates. SIMS measurements of metal substrates were also performed. The ability and limits of SIMS in the reactivity study of thallium oxide powders and films and, in addition, in the identification of reaction products evidencing impurity species that, in turn, can be ascribed to the substrates or to the precursors used for the oxide synthesis is pointed out.     

Evaporation‐Directed Crack‐Patterning of Metal–Organic Framework Colloidal Films and Their Application as Photonic Sensors

A straightforward crack‐patterning method is reported allowing the direct formation of periodic cracks in metal–organic framework (MOF) nanoparticle films during dip‐coating deposition. The crack propagation and periodicity can be easily tailored by controlling the evaporation front and the withdrawal speed. Several MOF‐patterned films can be fabricated on large surfaces and on several substrates (flat, curved or flexible) including the inner surface of a tube, not achievable by other lithographic techniques. We demonstrate that the periodic cracked arrays diffract light and, due to the MOF sorption properties, photonic vapor sensors are fabricated. A new concept of “in‐tube”, MOF‐based diffraction grating sensors is proposed with outstanding sensitivity that can be easily tuned “on‐demand” as function of the desired detection range.     

Lead Sulfide Nanocrystal-Polymer Composites for Optoelectronic Applications

Summary: Nanocomposite films were prepared by two methods in which lead sulfide (PbS) nanocrystals were contained in an organic matrix. One method used a wet chemical synthesis of the nanocrystals in the direct presence of a polymer, where the polymer controlled nanocrystal growth. The second method was gaseous deposition of nanocrystals into the organic phase. The two methods were similar in that the nanocrystals in the composites were free from surfactant capping layers that otherwise would add an interfacial region between the nanocrystal and the organic matrix. The gaseous deposition technique had several advantages over the wet chemical synthesis in that it allowed direct control over nanocrystal size and density, improved flexibility in the choice of organic phase, and was compatible with lithographic methods.     

Nanoscale patterning of two metals on silicon surfaces using an ABC triblock copolymer template

Patterning technologically important semiconductor interfaces with nanoscale metal films is important for applications such as metallic interconnects and sensing applications. Self-assembling block copolymer templates are utilized to pattern an aqueous metal reduction reaction, galvanic displacement, on silicon surfaces. Utilization of a triblock copolymer monolayer film, polystyrene-block-poly(2-vinylpyridine)-block-poly(ethylene oxide) (PS-b-P2VP-b-PEO), with two blocks capable of selective transport of different metal complexes to the surface (PEO and P2VP), allows for chemical discrimination and nanoscale patterning. Different regions of the self-assembled structure discriminate between metal complexes at the silicon surface, at which time they undergo the spontaneous reaction at the interface. Gold deposition from gold(III) compounds such as HAuCl4(aq) in the presence of hydrofluoric acid mirrors the parent block copolymer core structure, whereas silver deposition from Ag(I) salts such as AgNO3(aq) does the opposite, localizing exclusively under the corona. By carrying out gold deposition first and silver second, sub-100-nm gold features surrounded by silver films can be produced. The chemical selectivity was extended to other metals, including copper, palladium, and platinum. The interfaces were characterized by a variety of methods, including scanning electron microscopy, scanning Auger microscopy, X-ray photoelectron spectroscopy, and atomic force microscopy.     

贵金属单原子催化剂的制备及其在CO、VOCs完全氧化反应中的应用北大核心CSCD

为有效提高负载型催化剂中贵金属的原子利用效率,贵金属单原子催化剂逐渐成为一个研究热点和前沿课题.我们针对单原子催化剂在催化氧化领域中的应用,综述了几种贵金属单原子催化剂的典型制备方法,包括原子层沉积法、湿法化学法、光化学辅助法、热解法等,并讨论了上述方法的优缺点.此外,对比传统贵金属负载型催化剂,我们重点讨论了贵金属基单原子催化剂在CO催化氧化、挥发性有机化合物(VOCs)催化氧化、催化机理等催化氧化过程中的最新研究进展,尤其是贵金属基单原子催化剂在低温低浓度催化氧化过程中表现出的优异催化活性、抗水性和抗毒性,表明该类催化剂具备极大的工业应用潜力.最后,进一步从大规模工业应用角度探讨了单原子催化剂目前面临的挑战和可能的解决办法,期望可以为应用于催化氧化过程的高效、稳定的单原子催化剂的设计提供思路.     

Gold electroless deposition into poly-3,4-ethylenedioxythiophene films

By electroless deposition of gold into poly-3,4-ethylenedioxythiophene (PEDOT) films, the composite films (PEDOT-Au) are synthesized. Their electrochemical properties are studied by cyclic voltammetric (CVA) method. It is shown that in contrast to the original PEDOT film, the CVA curves of composite PEDOT-Au films measured in the presence of chloride ions reveal additional redox peaks associated with the presence of gold particles. The loading of metal gold particles by its chemical deposition into the polymer film is quantitatively assessed using quartz crystal microbalance method. The film mass is shown to depend on the time of gold loading and its original concentration in solution. The gold particles are shown to be oxidized by a reaction of the first order with respect to chloride ions. Based on the results of voltammetric and microbalance methods, the formation of a poorly soluble gold oxidation product Au(I)Cl in chloride-containing solutions was inferred.     

Study on the applications of SiC thin films to MEMS techniques through a fabrication process of cantilevers

We have tried to find the most suitable conditions for the deposition process of silicon carbide thin films as a material for MEMS techniques. We have also studied its application to semiconductor processes. To do this, we have tried to fabricate several dimensions of cantilevers with these silicon carbide thin films. High quality silicon carbide thin films are grown by metal-organic chemical vapor deposition (MOCVD). This process employs single molecular precursors such as diethylmethylsilane (DEMS), 1,3-disilabutane (DSB) at a pressure of 1 × 10⁻³ Pa and a growth temperature in the range of 700–1000 °C. Two fabrication methods are tested for initial fabrication of cantilevers. First, deposit SiC thin films on Si based atomic force microscopy (AFM) cantilevers. Second, used the lift-off process. To get three-dimensional cantilever-shaped SiC thin films, moreover, we chemically etched silicon substrate with strong alkaline solution such as TMAH at 80 °C. In addition, a high resolution of probe tips on the cantilevers was achieved using electron-beam deposition in a carbon atmosphere.     

Electrochemically fabricated molecule–electrode contacts for molecular electronics

Connecting molecules to electrodes is key for a range of applications. Conventional methods typically involve a spontaneous reaction of thiol/disulfide-terminated molecules with metal surfaces. Although modifying metal surfaces with thiol chemistry is simple, it is limited to forming a specific S–metal bonding, which is labile and hence there are concerns regarding its mechanical instability. In addition, spontaneous grafting requires long processing times to achieve high molecular coverages on the surface, which adds challenges for manufacturing devices comprising molecular films. Electrochemical methods for forming molecular films on surfaces offer powerful advantages over traditional methods, including reaction acceleration, molecular coverage control, and guiding the chemical bonding at the molecule?electrode interface. Electrochemical grafting enables connecting molecules to various types of electrodes including those that cannot be functionalized by other methods. More recently, electrochemical approaches were expanded to enable connecting 2D materials to electrodes, opening a realm of possibilities for hybrid technologies. In this opinion, we survey the recent progress in electrochemical methods for connecting (bio) molecules to electrodes for advancing molecular and bioelectronics.     

Area Selective Polymer Brush Deposition

Polymer brush films with chemical functionality to attach to site specific substrate areas are introduced for area selective deposition (ASD) application. It is demonstrated that polymer brushes with chemically defined end sites can be selectively bound to copper‐specific regions of patterned copper/silica (Cu/SiO₂) substrates. The process described overcomes various limitations of currently used technology including cost, complexity, and throughput, with potential implications for future electronic devices and nanomanufacturing. A comparative study of amine‐terminated polystyrene and amine‐terminated poly‐2‐vinyl pyridine polymer brushes (i.e., PS‐NH₂ and P2VP‐NH₂) with similar molecular weights display contrasting behavior on patterned Cu/SiO₂ line features. Further, a thiol terminated poly‐2‐vinyl pyridine polymer brush (i.e., P2VP‐SH) is investigated as a direct spin‐on process to fabricate a metal oxide layer atop Cu areas only. The results presented here detail a novel methodology and open a new exciting process for ASD practices that can facilitate the precise deposition of dense metal, semiconductor, or dielectric films. We also discuss the applicability of polymer brushes to ASD uses going forward.     

金属氧化物与现代微电子学:过渡金属前体化合物及转化为材料的化学过程

金属氧化物薄膜如HfO₂（被称为高k电介质）是现代微电子器件的关键组件,广泛用于计算机（平板电脑,笔记本电脑和台式机）、智能电话、智能电视、汽车和医疗设备中。具有大介电常数（k）的金属氧化物已经取代了介电常数小的SiO₂（k=3.9）,从而使得微电子元件进一步小型化。过渡金属化合物在化学气相沉积（CVD）和原子层沉积（ALD）中被广泛用作前体,通过与O₂、H₂O或O₃的反应生成金属氧化物薄膜。微电子金属氧化物膜是纳米材料最广泛应用的一个领域。本文概观该领域的最新进展,包括我们对d⁰过渡金属配合物与O₂反应的研究。     

溶液电沉积法制备苝酰亚胺类化合物薄膜

溶液电沉积法是一种具有沉积时间短,可以常温沉积以及沉积物在基底上附着力高等优点的薄膜制备方法.本文用水合肼增溶苝酰亚胺类化合物(PTCDI)的方法制备了可用于溶液电沉积的苝酰亚胺类化合物溶液.用紫外-可见分光光度法(UV-Vis)对溶解的过程进行了表征,并用顺磁共振(ESR)验证了水合肼对PTCDI的增溶过程实质上是化学反应过程.在制备苝酰亚胺类化合物溶液的基础上,采用阳极电沉积法在ITO导电玻璃上沉积出了薄膜.采用UV-Vis,扫描电镜(SEM)和X射线衍射仪(XRD)对薄膜进行了表征,证实得到了表面较为平整、具有可控结晶结构和形貌的PTCDI薄膜以及具有较宽光谱吸收范围的复合薄膜.     

金属氧化物与现代微电子学——过渡金属前体化合物及转化为材料的化学过程

金属氧化物薄膜如HfO₂（被称为高k电介质）是现代微电子器件的关键组件,广泛用于计算机（平板电脑,笔记本电脑和台式机）、智能电话、智能电视、汽车和医疗设备中。具有大介电常数（k）的金属氧化物已经取代了介电常数小的SiO₂（k=3.9）,从而使得微电子元件进一步小型化。过渡金属化合物在化学气相沉积（CVD）和原子层沉积（ALD）中被广泛用作前体,通过与O₂、H₂O或O₃的反应生成金属氧化物薄膜。微电子金属氧化物膜是纳米材料最广泛应用的一个领域。本文概观该领域的最新进展,包括我们对d⁰过渡金属配合物与O₂反应的研究。     

Synthesis of silver and copper nanoparticle containing a-C:Hby ion irradiation of polymers

One of the many applications of diamond like carbon (DLC) is the biocompatible coating of medical tools and implants. The most recent field of interest concerns the generation of antimicrobial activity in combination with the excellent wear resistance and biocompatibility of DLC. As has already been shown for polymers, nanoparticles of silver or copper within a carbonacious matrix have a bactericidal effect.In this work we describe a new technique to produce amorphous hydrogenated carbon films (a-C:H), which contain nanometer sized clusters of silver or copper. The hybrid deposition process is based on sol–gel synthesis of polymer films and subsequent ion-induced densification and cross-linking to form a-C:H. By thermal or UV-induced reduction of metal salts in polymer solution, colloidal metal particles are produced. In this way polymer films, already containing noble metal nanoclusters, can be deposited in a wet chemical step. Upon sol–gel deposition, the polymer is subjected to ion irradiation. Based on earlier results, the influence of ion energy on chemical and mechanical properties, as well as bonding structure is investigated. Special attention is also dedicated to ion-induced diffusion and growth as well as oxidation effects.     

Recent development in nanomaterials fabricated paper-based colorimetric and fluorescent sensors: A review

The paper-based sensing devices have drawn a broad interest in analytical chemistry for colorimetric and fluorescent-based analysis of biological, environmental, clinical, and food samples. It is due to the simple, rapid, biodegradable, user-friendly, less expensive, and low waste generation into the environment. Here, the recent development of paper-based sensors fabricated with different noble metal nanoparticles (NPs) and semiconductor and carbon quantum dots (QDs) is demonstrated to analyze several chemical substances from various samples. User-friendly and portable recording devices such as digital cameras, smartphones, scanners, etc. along with color detecting softwares are employed to measure the color intensity of nanomaterials fabricated paper devices after the deposition of a sample solution containing various chemical substances. The advantages and disadvantages of incorporating nanomaterials in the paper substrate (direct deposition, inkjet printing, screen printing and wax printing) are illustrated. The mechanism for colorimetric, fluorescence, phosphorescence, and chemiluminescence sensing using noble metal NPs (Ag, Cu, and Au), semiconductors, and carbon QDs for the determination of metal ions, anions, pesticides, biomolecules, and other toxic chemical substances are discussed. Thus, this review article would be highly useful for scientists and researchers to design colorimetric sensors to monitor chemical toxicants in clinical, environment, foods, and many other related samples.