ALD2018 Session NS-TuP: Nanostructures Synthesis and Fabrication Poster Session

Charge transport properties of organic semiconductors critically depend on their molecular packing structures. Controlling the charge transport by varying the molecular packing and understanding their structure−property correlations are essential for developing high-performance organic electronic devices. Here, we demonstrate that the charge carrier mobility in organic single-crystal nanowires can be modulated with respect to the intermolecular center-to-center distance by applying uniaxial strain to the cofacially stacked crystals. Furthermore, the measured values of charge carrier mobility were in good agreement with theoretical calculations based on charge localized hopping theory. These results provide a definitive relationship between intermolecular packing arrangement and charge transports, which enables a huge improvement in charge carrier mobility for organic single-crystal materials.

A titanium-aryloxide (Ti-Ao) thin films showing high visible-light absorption was fabricated by molecular layer deposition (MLD). The origin of the visible light absorption was ligand-to-metal charge-transfer that is revealed by studying ultraviolet-visible analysis and density functional theory calculations. ZnO/Ti-Ao nano-laminate active channel photo-transistors functioned synergistically with Ti-Ao acting as a visible-light absorber, and ZnO serving as a great semiconductor.

Carbon nanofibers (CNF) are used as active materials for electrodes in various energy devices, such as lithium ion secondary batteries, super capacitors and fuel cells. Recent studies have been proved that nanoscale coatings on carbon nanotubes increase the output and lifespan of the devices owing to the improvement of mechanical and chemical properties. Among the various coating methods, atomic layer deposition (ALD) can conformally adjust the thickness of coating layer without any directional growth. Therefore, it has the advantage of coating on the particle with high aspect ratio, such as CNF, even in nanometers thickness.

In this work, we grow SnO₂ layers having two kinds of morphology on CNF. We compare the reactivity and wettability of precursors and reactant gases as follows; trimethylaluminum (TMA), tetrakis(dimethylamino)tin (TDMASn), Sn(dmamp)₂, H₂O, O₃. Since the ALD reaction of TMA, Sn(dmamp)₂, O₃ with surface functional group requires the smallest activation energy, it is easier to react with surface functional sites. We also use two types of ALD equipment; Flow-type ALD (static ALD), and fluidized bed reactor type ALD (dynamic ALD). Static ALD is processed at 161℃, 5 cycles by TMA and H₂O as Al₂O₃ reactant, and then followed by 105 cycles of TDMASn and H₂O as SnO₂ reactant. Dynamic ALD is processed at 120℃, 5 cycles by TMA and H₂O as Al₂O₃ reactant, and 150℃ 105 cycles by TDMASn and H₂O as SnO₂ reactant.

According to the TEM analysis, static ALD creates an island-shaped SnO₂, but uniform SnO₂ layer is formed by inserting Al₂O₃ layer. On the contrary, dynamic ALD shows formation of uniform SnO₂ layer irrespective of Al₂O₃ layer. In addition, energy dispersive spectroscopy (EDS) analysis showed more intensive elemental Sn and O signal can be detected from Al₂O₃-coated CNF. XPS analysis reveals that both Sn⁴⁺ and Sn²⁺ are observed in SnO₂ on CNF by static ALD probably due to the formation of an interfacial layer between the SnO₂/CNF interface. Meanwhile, Sn⁴⁺ spectra are only shown in SnO₂ on CNF by dynamic ALD. Cyclic voltammetry analysis is performed to characterize the electrochemical properties of SnO₂ coated CNF as an electrode on fuel cell.

In recent years, transition metal oxides and dichalcogenides have received much attention due to its attractive properties for a wide range of applications. Among these materials molybdenum compounds were studied most initiatively and achieved considerable progress. Meanwhile, metallic molybdenum is also widely used as conducting materials in many electronic applications. In this study, therefore, optical, structural and electronic properties of metallic molybdenum and its compounds thin films have been investigated. Plasma-enhanced atomic layer deposition (PEALD) was employed to form the uniform Mo, MoO₃, MoS₂ thin films. Especially, When Mo(CO)₆ was used as the precursor, different kinds of plasma gases such as O₂, H₂, H₂S and their mixtures for reactant resulted in selective growth of Mo, MoO₃, MoS₂, respectively. Basically, the ALD characteristics with each reactant were studied. The ellipsometry, raman spectroscopy, photo luminescence, X-ray photoelectron spectroscopy, scanning electron microscopy were used to examine film characteristics according to the different precursor combinations and growth conditions. Furthermore, the potential of metallic Mo as well as Mo compounds for device component was investigated.

An ammonia gas sensor which was fabricated based on the P3HT organic field-effect transistor (OFET) in the range of 0.01 to 25ppm. Single-crystal poly(3-hexylthiophene) (P3HT) nanowires (100 nm wide 150 nm high nanowires that were separated a 600nm space) were used as the organic semiconductor (OSC) layer of an OFET with a top-contact geometry. This ammonia gas sensor showed outstanding results such as electrical and sensitivity characteristics. First, electrical properties (high mobility, output signal and on/off ratio) of single crystal P3HT nanowire OFET were around two orders of magnitude higher than those of P3HT thin film OFET. Second, it showed enhanced ammonia sensitivity, around three times higher than conventional P3HT thin film OFET with same geometry. In addition, the reversibility of ammonia response was presented on this OFET, and constant results were obtained in the changes of humidity from 45% to 100%. These excellent performances of P3HT nanowire were presented due to high surface/volume ratio and single crystal characteristics of the P3HT nano wires.

Due to increased research interest in 2-dimensional nanomaterials, single- and multi-layer transition metal dichalcogenide (TMD) nanosheets have attracted considerable attention for their potential use in the fields of catalysis, electronic switching devices, and gas sensors. [1] The key technology for utilizing TMD in various applications is a reliable fabrication of the nanosheets over a large area. Chemical vapor deposition and sulfurization have been introduced for large-area synthesis, but the high-temperature conditions adopted in these methods are not compatible with many of the device fabrication processes, especially on flexible substrates. Therefore, low-temperature atomic layer deposition (ALD) process using highly reactive precursors has been suggested as an alternative to overcome the thermal limit and to enable the practical use of TMD for electronic devices. [2]

In this report, TiTe₂ thin films were deposited by ALD, adopting TiCl₄ and [(CH₃)₃Si]₂Te as the Ti- and Te-precursors, respectively. Thermally activated ligand exchange occurred at a substrate temperature of 60~200 C, and the saturated growth rate (at 180^oC) was ~15 ng∙cm^-2cy^-1 as shown in Figure 1. T he ALD TiTe₂ films grown over 180^oC showed extraordinary plate-shaped hexagonal TiTe₂ grains, parallel to the substrate surface, on Si substrate. They were aligned along the (001) plane, showing sharp (001) family peaks in theta-two theta X-ray diffraction (XRD) as shown in figure 2. Further confirmations of layered growth of the deposited TiTe₂ are analyzed by scanning electron microscopy (SEM) and Raman spectroscopy . Tilted and vertically grown plate-shaped grains were confirmed in SEM, but the amount was small, showing no peak in grazing-angle X-ray diffraction (GAXRD). This preference was more pronounced for those deposited on sapphire (0001) substrates, which is attributed to the low atomic mismatch of the same hexagonal crystal structure of sapphire and TiTe₂. A bottom-gate thin film transistor (TFT) structure was used to characterize the electrical properties of TiTe₂ TFTs. Figure 3 displays the transfer curves of the TFT, showing a typical p-type conduction behavior. Although the demonstrated device performance is quite low compared to other p-type oxide TFTs, these results demonstrate the feasibility of the atomic layer deposited TMD TFT. Further detailed discussions on such ALD deposition will be made in the presentation.

[1] J. A. Wilson et al., Adv. Phys., 18, 193-335 (1969)

[2] I.H. Baek et al., Chem. Mater., 29 (19), 8100-8110 (2017)

In this study, we report conductivity switches from p-type of BiOCl to n-type of black BiOCl with generated oxygen vacancies by UV illumination at the exposed (001) surface. This n-type conductivity occurs by formation of oxygen vacancy at the surface of BiOCl nanosheets, confirmed by XPS spectra that oxygen vacancy gradually increasing as a function of UV illumination time. The raman spectra shows oxygen vacancy related vibration mode was rising under 2 mtorr and low temperature PL, the result reveals decreasing band edge emission of BiOCl due to high density oxygen vacancies placed under conduction band edge of black BiOCl. The oxygen partial pressure dependent electrical conductivity showsσ=pO₂^1/4.05 with a positive slope under low pressure region for the BiOCl and σ=pO₂^1/5 with a negative slope under atmospheric region for the black BiOCl.

Metal Oxide Semiconductor (MOS) gas sensors have received significantly attentions as electronic devices for the specifics identification and concentration detection of combustible and hazardous gases. Among various semiconductor materials used in MOS gas sensors, ZnO has been widely used for gas sensor applications due to its good electrical conductivity, wide band gape of 3.37 eV, ~60 meV exciton binding energy, low cost, and high mechanical stability. Currently ZnO ethanol sensors based on nanorod structures have been extensively investigated due to their high electrochemical stability, suitable doping, nontoxicity, and high surface-to-volume ratio. ZnO gas sensors with one-dimensional nanostructures have good performance in terms of their sensing response, response time and recovery time.

In this project, Aluminum doped ZnO (AZO) thin films coated by Atomic Layer Deposition (ALD) was deposited on the surface of the ZnO nanostructure to further enhance its sensing performance. The ZnO seed layers were deposited on Si wafer by ALD with (C₂H₅)₂Zn and DI water as precursors of zinc and oxygen. Hydrothermal method was carried out to grow aligned ZnO nanorods. There was a layer of Al₂O₃ as sacrificial spacer deposited on the surface of ZnO nanorods by ALD with Al₂(CH₃)₆ and DI water as precursors followed by an additional layer of ZnO thin film synthesized by ALD. To expose the Al₂O₃ sacrificial spacer layer, Precision Ion Polishing System was employed to remove the top cover of synthesized ZnO nanorods. Then Sodium hydroxide was applied to etch the Al₂O₃ layer to form ZnO nanorod/nanotube structure. Finally, Al doped ZnO thin films were deposited on the surface of ZnO nanorod/nanotube by ALD to further enhance its sensing performance to ethanol vapors.

The sensing performance of ZnO nanorod/nanotube to ethanol vapor was investigated by a gas sensor testing system which was home-made with a sealed reaction chamber, ethanol vapor generator, testing circuit, and controlled heating system. The gas sensor testing system interface is operated in LabVIEW program and CompactRio system from National Instruments. The resistant variances of ZnO nanorod/nanotube were measured by the home-made sensor testing system. The demonstrated sensing performance results include the sensing response comparison between ZnO nanorods and ZnO nanorod/nanotube structures before and after ALD coatings with nested AZO films at different temperatures and with various concentrations of input ethanol vapor. The response times and recovery times of ZnO nanorod/ nanotube samples before and after ALD coatings with nested AZO thin films were analyzed to investigate the sensing enhancement.

Rotary ALD, wherein oxide and nitride shells are grown by ALD on nanoparticles agitated in a rotating drum, is an emerging technique for scalable production of a wide variety of core/shell nanoparticles. Due to deposition temperature limitations, the shells are typically grown in the amorphous phase. However, many applications of these particles, including batteries and structural nanocomposites, require crystalline shells. In this work we study the crystallization of ALD-grown shells by post-deposition thermal annealing, an approach rarely examined in detail in the nanoparticle ALD literature, despite the technological potential of fully crystalline core/shell nanoparticles. The crystallization temperature of nanoshells is difficult to predict due to a variety of competing effects of spheroidal geometry. For example, compared to a planar film of the same thickness, a nanoshell has more atoms at the surface but fewer atoms at the film/substrate interface. Using Al₂O₃ shells as a prototype, we investigate the dependence of crystallization behavior on shell thickness as well as underlying nanoparticle material and size. We characterize shell composition, structure, and morphology by XPS, XRD, and TEM.

Carbon nanotubes (CNTs) coated with titanium dioxide (TiO₂) have become a promising hybrid nanomaterial for a wide range of applications, e.g. photocatalysis, gas sensors, dye-sensitized solar cells and energy storage. These nanostructures combine the high surface area and excellent thermal and electrical conductivity of the CNTs with the high photoactivity and chemical stability of TiO₂. In this study multi-walled carbon nanotubes (MWCNTs) were grown by catalytic chemical vapor deposition on Si and Si/SiO₂ substrates using Fe catalyst. They were subsequently coated by atomic layer deposition (ALD) of TiO₂ from tetrakis(dimethylamido)titanium. Since the ALD is a self-limiting surface process it is important to understand how it proceeds on the inert and nanostructured surface of carbon nanotube forest. Therefore, we tested different plasma modifications of MWCNTs surface (oxygen plasma treatment, carboxyl or amine plasma enhanced chemical vapor deposition) prior to the ALD and also compared thermal and plasma enhanced ALD of TiO₂. Pristine and modified MWCNTs were characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy.

DNA nanostructures are versatile templates for low cost nanofabrication. However, their applications in nanofabrication have long been limited to low temperature processes. We show that a thin film of Al₂O₃, grown by ALD, enables the high temperature cabonization of DNA into carbon nanostructures. Atomic force microscopy (AFM) data showed that carbon nanostructures preserved the shape of the DNA nanostructure. Conductive AFM measurement shows that the carbon nanostructures are electrically conductive.

Industrial applications of nanoparticles have surged recently. In most cases, however, there are difficulties in utilizing nanoparticles itself without any further processing. For example, protective layer coating on metal nanoparticles should be processed to circumvent explosive combustion reaction. Thus, need for development of multipurpose nanoparticles coating techniques, has brought great enthusiasm among researchers. Atomic layer deposition (ALD), a vapor deposition technique, has potential to produce conformal and uniform ultrathin-film on nanoparticles due to sequential self-limiting surface chemical reactions. Being capable of these unique merits, ALD can be introduced to applications including photocatalysts, cosmetics, optical quantum dots and protective layers for various nanoparticles/nanostructures. Herein, we used specially-designed ALD rotary reactor in order to address critical issues in cosmetics and photocatalysis applications. Particularly, in photocatalysis, we deposited TiO₂ and ZnS on g-C₃N₄ to reduce recombination rate of charge carriers. In addition, we studied anti-photocorrosion effect of TiO₂ passivation layer on ZnO nanoparticles. In cosmetics application, study on Al₂O₃ blocking barrier for phototoxicity of TiO₂ particles as a sunscreen ingredient was carried out. It was demonstrated that all the nanoparticles with ALD coating layer showed superior properties. Details in each work will be discussed.