Differential mobility analysis of nanoparticles generated by laser vaporization and controlled condensation (LVCC)

Silicon and iron aluminide (FeAl) nanoparticles were synthesized by a laser vaporization controlled condensation (LVCC) method. The particles generated by the laser ablation of solid targets were transported and deposited in the presence of well-defined thermal and electric field in a newly designed flow-type LVCC chamber. The deposition process of nanoparticles was controlled by the balance of the external forces; i.e., gas flow, thermophoretic and electrostatic forces. The size distributions of generated nanoparticles were analyzed using a low-pressure differential mobility analyzer (LP-DMA). The effect of synthesis condition on the size distribution was analyzed by changing the pressure of the carrier gas (20–200 Torr), the temperature gradient in the LVCC chamber (ΔT=0–190°C) and the electric field applied between the LVCC chamber plates (E=0–3000 V/m). It was found that electrostatic field was effective to selectively deposit small size nanoparticles (about 10 nm) with expelling large droplet-like particles.     

Classification Performance of a Low Pressure Differential Mobility Analyzer for Nanometer-sized Particles

Size distribution measurements and classification tests by a low pressure differential mobility analyzer (LPDMA) for nanometer-sized silver particles and cesium iodide particles under low pressure conditions (123–300 Torr) were performed using a transmission electron microscope (TEM) and the tandem DMA technique. When the ratio of the sheath gas flow rate and the aerosol gas flow rate was set at 5 : 1, the targeted sizes calculated from the classification voltage applied to the LPDMA at 160 Torr are found to be in good correlation with the count mean Feret diameter obtained from the TEM observation of the particles collected after a classification ranging from 6 to 25 nm, although the targeted sizes set by the LPDMA were approximately 15% greater than the count mean Feret diameter measured by the TEM after classification. The geometric standard deviations of DMA-classified particles measured by TEM and those obtained from the tandem DMA method ranged from 1.08 to 1.17 and from 1.05 to 1.13, respectively. They were slightly greater than the ideal geometric standard deviations (1.05) of the particles classified with the LPDMA, which was calculated by neglecting the broadening effects due to Brownian diffusion. We experimentally demonstrated the validity of our LPDMA system for size measurements and classification of the nanometer-sized particles under low pressure conditions.     

Control of nanoparticle size and amount by using the mesh grid and applying DC-bias to the substrate in silane ICP-CVD process

The effect of applying a bias to the substrate on the size and amount of charged crystalline silicon nanoparticles deposited on the substrate was investigated in the inductively coupled plasma chemical vapor deposition process. By inserting the grounded grid with meshes above the substrate, the region just above the substrate was separated from the plasma. Thereby, crystalline Si nanoparticles formed by the gas-phase reaction in the plasma could be deposited directly on the substrate, successfully avoiding the formation of a film. Moreover, the size and the amount of deposited nanoparticles could be changed by applying direct current bias to the substrate. When the grid of 1 × 1-mm-sized mesh was used, the nanoparticle flux was increased as the negative substrate bias increased from 0 to – 50 V. On the other hand, when a positive bias was applied to the substrate, Si nanoparticles were not deposited at all. Regardless of substrate bias voltages, the most frequently observed nanoparticles synthesized with the grid of 1 × 1-mm-sized mesh had the size range of 10–12 nm in common. When the square mesh grid of 2-mm size was used, as the substrate bias was increased from – 50 to 50 V, the size of the nanoparticles observed most frequently increased from the range of 8–10 to 40–45 nm but the amount that was deposited on the substrate decreased.     

Formation process of Si nanoparticles in rare gas observed by a decomposition method

We have investigated the formation process of silicon nanoparticles after laser ablation of silicon targets in argon gas. The nanoparticles exhibit bright photoluminescence in the visible wavelength range and can be applied to opto-electronic devices. In order to observe silicon nanoparticles, we have developed a decomposition method. The nanoparticles were probed by detecting light emission resulting from decomposition using a second laser. This method enables us to observe nanoparticles that cannot be observed directly by the methods applied so far. We have observed that the nanoparticles grow in time periods of 1.0-1.8 ms following ablation in Ar gas at 5 Torr when Si targets are ablated at 5 J/cm² with a pulse width of 7 ns. The nanoparticles begin to grow above ablation spots slightly apart from the targets just after thermalization of the plume. We also found that the growth is delayed at higher fluxes of ablation laser light.     

Synthesis of single-crystal Sm-Co nanoparticles by cluster beam deposition

Single-crystal Sm-Co nanoparticles have been successfully produced by a cluster beam deposition technique. Particles have been deposited by DC magnetron sputtering using high Ar pressures on both single-crystal Si substrates and Au grids for the magnetic and structural/microstructural properties, respectively. Oxidation of the particles is prevented by using carbon buffer and cover layers. Nanoparticles have a uniform size distribution with an average size of 4.2, 6 and 7 nm at 1, 1.5 and 2 Torr of Ar pressure, respectively. At 1 Torr, the particles have the disordered 1:7 structure and a high coercivity of 19 kOe at 10 K. These particles show a superparamagnetic behavior with a blocking temperature of T_B = 145 K. From this value of T_B and the particle volume, the value of anisotropy constant K is estimated to be around 2.2 × 10⁷ ergs/cc. Heat is introduced to the particles during their flight to the substrate to increase the particle size. Nanoparticles of SmCo₅ with an average size of 15 nm and high room temperature coercivity have been produced. No change in magnetic and structural properties of the samples has been observed even after 10 months. Cluster beam deposition could play a key role for the production of rare earth nanoparticles for many applications.     

外加气流对脉冲激光烧蚀制备纳米Si晶粒尺寸分布的影响

提出一种控制脉冲激光烧蚀制备纳米Si晶粒尺寸分布的新方法。在10Pa的Ar环境中,采用脉冲激光烧蚀高阻抗单晶硅靶沉积制备了纳米Si晶薄膜。在羽辉正上方2.0cm,距靶0.3～3.0cm范围内的不同位置引入氩气流,在烧蚀点正下方2.0cm处水平放置单晶Si(111)衬底来收集制备的纳米Si晶粒。利用扫描电子显微镜观察样品表面形貌,并对衬底不同位置上纳米Si晶粒进行统计。结果表明:在不引入气流时,晶粒的尺寸随靶衬间距的增加先增大后减小,晶粒尺寸峰值出现在距靶1.7cm处;引入气流后,晶粒尺寸分布发生变化,在距靶1.7cm引入气流时晶粒尺寸峰值最大,在距靶3.0cm引入气流时晶粒尺寸峰值最小,且出现晶粒尺寸峰值的位置随着引入气流位置的增加而增大。     

Field evaporation of silicon surfaces

Quantitative measurements on field evaporation of Si(111) surfaces in hydrogen imaging gas have been carried out by field ion microscopy. The field evaporation rate is found to increase exponentially with increase of the reciprocal of tip temperature in the range 80–103 K. The evaporation field strength increases with increase of tip temperature in the investigated range, 80–300 K. Within the applied pressure range, 5× 10^?6 to 2 × 10^?4 Torr of hydrogen gas, the evaporation rate linearly increases with the gas pressure. Similar effects of temperature and gas pressure on field evaporation of Si(111) surfaces have been observed also in silane imaging gas. A model, based on a field-induced formation of surface hydrides as a rate-determining step, is proposed, which accounts for all the experimental results of the field evaporation process.     

In situ size measurement of Si nanoparticles and formation dynamics after laser ablation

We have developed a technique to detect Si nanoparticles selectively and to measure size in situ. Applying the technique, we have investigated formation process of Si nanoparticles after pulsed laser ablation of Si targets in Ar gas. Time-resolved photoluminescence (PL) spectroscopy revealed that PL only from Si nanoparticles is observed below 2.4 eV while PL from Si nanoparticles as well as defects in SiO₂ is observed above 2.4 eV. Therefore, Si nanoparticles can be detected selectively by excitation light with a photon energy below 2.4 eV. It is found that the onset of the PL from Si nanoparticles is delayed by approximately 0.3 ms from that of the defects and smaller Si nanoparticles. A size can be estimated by a band gap, which is roughly equal to the lowest photon energy at which Si nanoparticles can be excited. Thus, we estimated the sizes of growing Si nanoparticles. PACS 61.46.+w; 78.66.w; 07.60.Yi     

A new dual-type DMA for measuring nanoparticles emitted from combustion engines

A new dual-type differential mobility analyzer (dual-type DMA) was developed in order to detect transient number concentrations of airborne nanoparticles with diameters centralized at around 10 nm (for nuclei mode particles) and 100 nm (for accumulation mode particles) in automobile exhaust gas. The apparatus divides the gas sample into two parts, and each part is sent through one of two coaxially nested sections for analysis. For the scanning mode measurement, the nanoparticles are charged by ²⁴¹Am and their size distributions are determined by varying the applied voltage over 2 min. The transient mode measurement, on the other hand, fixes the voltages for the two sections at peaks near 10 and 100 nm in order to monitor the transient behavior of the exhaust nanoparticles. The measurement principles and design of the dual-type DMA are detailed and the results for time response experiments are presented using model nanoparticles charged by a corona charger. The transient concentrations of the nuclei mode and the accumulation mode particles from a diesel engine were shown to be detected by this method, when ²⁴¹Am was used for charging the particles.     

Influence of substrate bias voltage on the microstructure of nc-SiOx:H film

Amorphous silicon oxide containing nanocrystalline silicon grain(nc-SiO_x:H) films are prepared by a plasmaenhanced chemical vapor deposition technique at different negative substrate bias voltages.The influence of the bias voltage applied to the substrate on the microstructure is investigated.The analysis of x-ray diffraction spectra evidences the in situ growth of nanocrystalline Si.The grain size can be well controlled by varying the substrate bias voltage,and the largest size is obtained at 60 V.Fourier transform infrared spectra studies on the microstructure evolutions of the nc-SiO_x:H films suggest that the absorption peak intensities,which are related to the defect densities,can be well controlled.It can be attributed to the fact that the negative bias voltage provides a useful way to change the energies of the particles in the deposition process,which can provide sufficient driving force for the diffusion and movement for the species on the growing surface and effectively passivate the dangling bonds.Also the larger grain size and lower band gap,which will result in better photosensitivity,can also be obtained with a moderate substrate bias voltage of 60 V.     

Effects of total CH4/Ar gas pressure on the structures and field electron emission properties of carbon nanomaterials grown by plasma-enhanced chemical vapor deposition

The effects of total CH₄/Ar gas pressure on the growth of carbon nanomaterials on Si (1 0 0) substrate covered with CoO nanoparticles, using plasma-enhanced chemical vapor deposition (PECVD), were investigated. The structures of obtained products were correlated with the total gas pressure and changed from pure carbon nanotubes (CNTs) through hybrid CNTs/graphene sheets (GSs), to pure GSs as the total gas pressure changed from 20 to 4 Torr. The total gas pressure influenced the density of hydrogen radicals and Ar ions in chamber, which in turn determined the degree of how CoO nanoparticles were deoxidized and ion bombardment energy that governed the final carbon nanomaterials. Moreover, the obtained hybrid CNTs/GSs exhibited a lower turn-on field (1.4 V/μm) emission, compared to either 2.7 V/μm for pure CNTs or 2.2 V/μm for pure GSs, at current density of 10 μA/cm².     

Vacuum versus gas environment for the synthesis of nanocomposite films by pulsed-laser deposition

Nanocomposite films formed by Cu nanocrystals (NCs) with sizes <10 nm embedded in an amorphous Al₂O₃ host have been grown by alternate pulsed-laser deposition both in vacuum and in a buffer gas (Ar) up to pressures of 0.1 Torr. The dimensions, dimension distributions, and shape of the NC produced in vacuum and in Ar up to pressures of 5᎒^-3 Torr follow a similar trend as a function of the Cu areal density. This allows us to conclude that the nucleation and growth of the NC are dominated by processes occurring at the substrate surface rather than in the gas phase. For Ar pressures ̓᎒^-2 Torr, the anisotropy of the NC is enhanced, the deposition rate decreases abruptly and a significant amount of the buffer gas is incorporated into the host, thus leading to the formation of a porous material.     

Synthesis of Si nanoparticles with narrow size distribution by pulsed laser ablation

We synthesized Si nanoparticles by pulsed nanosecond-laser ablation. We applied a positive voltage bias during laser irradiation and effectively reduced size distribution. Scanning electron micrographs of samples showed the nanoparticles to be highly non-agglomerated. Si nanoparticles have the average diameter of 4–5 nm, the geometrical standard deviation of 1.35, and the density of 1.6 × 10¹²/cm². A MOS device showed excellent charge trap behavior with a flat-band voltage shift over 7 V, which can be applied for memory device applications.     

A new dual-type DMA for measuring nanoparticles emitted from combustion engines

A new dual-type differential mobility analyzer (dual-type DMA) was developed in order to detect transient number concentrations of airborne nanoparticles with diameters centralized at around 10 nm (for nuclei mode particles) and 100 nm (for accumulation mode particles) in automobile exhaust gas. The apparatus divides the gas sample into two parts, and each part is sent through one of two coaxially nested sections for analysis. For the scanning mode measurement, the nanoparticles are charged by ²⁴¹Am and their size distributions are determined by varying the applied voltage over 2 min. The transient mode measurement, on the other hand, fixes the voltages for the two sections at peaks near 10 and 100 nm in order to monitor the transient behavior of the exhaust nanoparticles. The measurement principles and design of the dual-type DMA are detailed and the results for time response experiments are presented using model nanoparticles charged by a corona charger. The transient concentrations of the nuclei mode and the accumulation mode particles from a diesel engine were shown to be detected by this method, when ²⁴¹Am was used for charging the particles.This revised version was published online in August 2005 with a corrected issue number.     

Studies of diamond-like carbon （DLC） films deposited on stainless steel substrate with Si/SiC intermediate layers

In this work, diamond-like carbon （DLC） films were deposited on stainless steel substrates with Si/SiC intermediate layers by combining plasma enhanced sputtering physical vapour deposition （PEUMS-PVD） and microwave electron cyclotron resonance plasma enhanced chemical vapour deposition （MW-ECRPECVD） techniques. The influence of substrate negative self-bias voltage and Si target power on the structure and nano-mechanical behaviour of the DLC films were investigated by Raman spectroscopy, nano-indentation, and the film structural morphology by atomic force microscopy （AFM）. With the increase of deposition bias voltage, the G band shifted to higher wave-number and the integrated intensity ratio ID/IG increased. We considered these as evidences for the development of graphitization in the films. As the substrate negative self-bias voltage increased, particle bombardment function was enhanced and the sp^3-bond carbon density reducing, resulted in the peak values of hardness （H） and elastic modulus （E）. Silicon addition promoted the formation of sp^3 bonding and reduced the hardness. The incorporated Si atoms substituted sp^2- bond carbon atoms in ring structures, which promoted the formation of sp^3-bond. The structural transition from C-C to C-Si bonds resulted in relaxation of the residual stress which led to the decrease of internal stress and hardness. The results of AFM indicated that the films was dense and homogeneous, the roughness of the films was decreased due to the increase of substrate negative self-bias voltage and the Si target power.     

Fabrication and characterization of a granular film consisting of size-selected silver nanoparticles: application to a SERS substrate

Uniform-sized silver nanoparticles with average diameter of 13.7 nm have been prepared in the gas-phase by combining a pulsed laser ablation method with a low pressure-differential mobility analyzer (LP-DMA). By depositing the silver nanoparticles onto a silicon substrate, a granular film consisting of size-selected silver nanoparticles has been fabricated and its morphology and electronic properties have been examined using transmission electron microscopy (TEM) and UV-visible absorption spectroscopy. This granular silver film serves as a highly active substrate for surface-enhanced Raman scattering (SERS).     

Nanoparticle-Coated Silicon Nanowires

The synthesis of silicon nanowires that are densely coated with silicon nanoparticles is reported. These structures were produced in a two-step process, using a method known as hypersonic plasma particle deposition. In the first step, a Ti–Si nanoparticle film was deposited. In the second step the Ti-source was switched off, and nanoparticle-coated nanowires grew under the simultaneous action of Si vapor deposition and bombardment by Si nanoparticles. Total process time, including both steps, equaled 5 min, and resulted in formation of a dense network of randomly oriented nanowires covering1.5 cm² of substrate area. The nanowires are composed of single-crystal Si. The diameters of the nanowires vary over the range 100–800 nm. Each nanowire has a crystalline TiSi₂ catalyst particle, believed to have been solid during nanowire growth, at its tip.     

Magnetron and pulsed laser deposition of silver and gold nanoparticles and discontinuous films and their optical properties

Ag and Au nanoparticles are obtained by magnetron sputtering and pulsed laser deposition under different conditions, and the features of their absorption spectra associated with plasmon resonances are investigated. Optimal deposition conditions for obtaining small (5?C10 nm) silver nanoparticles with a high density of surface distribution include an increased argon pressure (2.5 × 10^?2 Torr) and a low discharge voltage (100 V). Gold nanoparticle arrays obtained by pulsed laser deposition at a temperature of 200°C in vacuum are more uniformly distributed on the substrates than those deposited at room temperature in argon. It is shown that the maximum of the plasmon absorption shifts toward shorter wavelengths with a decrease in the equivalent thickness of metal films and depends not only on this thickness but also on the type of substrate, which is responsible for the morphology of nanoparticle arrays.     

Synthesis of multi-walled carbon nanotubes for NH3 gas detection

It has been recently demonstrated that carbon nanotubes (CNTs) represent a new type of chemical sensor capable of detecting a small concentration of molecules such as CO, NO₂, NH₃.In this work, CNTs were synthesized by chemical vapor deposition (CVD) on the SiO₂/Si substrate by decomposition of acetylene (C₂H₂) on sputtered Ni catalyst nanoparticles. Their structural properties are studied by atomic force microscopy, high-resolution scanning electron microscopy (HRSEM) and Raman spectroscopy. The CNTs grown at 700 °C exhibit a low dispersion in size, are about 1 μm long and their average diameter varies in the range 25–60 nm as a function of the deposition time. We have shown that their diameter can be reduced either by annealing in oxygen environment or by growing at lower temperature (less than 600 °C).We developed a test device with interdigital Pt electrodes on an Al₂O₃ substrate in order to evaluate the CNTs-based gas sensor capabilities. We performed room temperature current–voltage measurements for various gas concentrations. The CNT films are found to exhibit a fast response and a high sensitivity to NH₃ gas.     

金刚石薄膜负偏压形核的边缘效应

 在微波等离子体化学气相沉积装置中，研究了金刚石薄膜在Si (100)面上的负偏压形核行为，结果表明，偏压大小对金刚石的形核均匀性有显著影响，而甲烷浓度主要影响形核时间，对金刚石的最大核密度影响不大。在硅片尺寸小于钼支撑架时，形核行为存在明显的边缘效应，即在偏压值低于-150 V时，硅片边缘金刚石核密度急剧降低，远低于硅片中央；在甲烷浓度比较低时，硅片边缘核密度要高于中间。研究表明，造成这种现象的主要原因是硅片下的钼支撑架发射电子所致，过量的原子H对金刚石的形核是不利的。