Session M12: Materials: Synthesis, Growth, Processing, and Defects

Nanoscale Mapping of Heterogeneous Strain and Defects in Individual Magnetic Nanocrystals

Coherent X-ray Diffractive Imaging (CDI), with refraction and elemental sensitivities, is one of the strongest contenders for investigating internal structures (both atomic density and phases) of nanocrystalline materials. The traditional high-resolution imaging techniques such as Transmission Electron Microscopy (TEM) require slicing the specimens to probing very thin cross-sections of the materials. Any internal stresses could possibly be removed when samples are cross-sectioned to produce thin lamellar sections. On the contrary, with CDI, we can probe the entire samples' three-dimensional (3D) internal structure with the native states of the specimens mostly preserved. CDI utilizes advanced iterative reconstruction algorithms in which solutions are to be iterated between real-space and reciprocal-space by forward Fourier transform and reverse Fourier transform, thus retrieving the converged solutions of real-space complex-valued sample wavefunction that satisfy the imposed constraints in both real and reciprocal-spaces. We performed studies on a single 240nm wide Ni/NiO nanowire of a core-shell morphology using Bragg CDI. We utilize the theory of dislocations to compare an individual dislocation obtained from Bragg CDI with theory, with the overall region of the reconstructed core–shell structure shown. The observed symmetrical (and inverse radial distance from dislocation line) decay of the displacement field is consistent with the theory of elasticity.

In conclusion, Bragg CDI is ideal for investigating single crystalline samples in conjunction with in-situ and operando capabilities in the current synchrotron radiation facilities worldwide. We envisage significant advancements in the fundamental and applied nanomaterials research using Bragg CDI shortly.   

Role of interfaces in materials synthesis under electromagnetic field

Advancements in synthesis science and the introduction of various nanostructured materials in the industry have stimulated fundamental research to the path for scalable and cost-effective manufacturing techniques. In response to the growing demand for the fabrication of dense industrial products, microwave and laser-assisted synthesis have expanded significantly due to their effectiveness in producing various materials with desired microstructures. The microstructure is primarily shaped by the interaction of the electromagnetic field with the interfaces at the surface of the films or inside the bulk materials. The ponderomotive force (PMF) and its role is modeled and quantified to explain the microstructure evolution under the electromagnetic field. The model illustrates that near the interfaces, the net direction of the PMF can conduct a mass transport toward or away from the interfaces to fill or deplete the pores depending on the field properties and the interface geometries. Moreover, it shows that PMF can facilitate the diffusion of mobile ionic species and accelerate the reaction between the constituent elements. The results contribute to our physical understanding and control of the field-assisted synthesis to develop reproducible and scalable production approaches.       

The effects of island morphology, insulator vacancies, and band bending on the electrical transport in YSZ- and SiNx-based granular metals

Granular metals (GMs) are a class of disordered conductor-insulator composites consisting of nanoscale metallic islands dispersed within a dielectric matrix. Between 30-50% metal concentration, by volume, the metal islands begin forming electrical pathways, and the GMs transition from a dielectric to a metallic regime. In the dielectric regime, GMs exhibit both thermally activated tunneling and capacitive transport. As such, GMs are of great interest for fundamental studies on electrical transport through discontinuous films. Despite over 50 years of GM research, almost all GM studies utilize SiO₂ or Al₂O₃ combined with a low temperature stability metal. Additionally, the interfacial metal-insulator interactions have not been studied. Here, we report the synthesis of GMs through co-sputtering of Co or Mo with YSZ or SiN_x, with volumetric metal concentrations ranging from 20% to 80%. Transmission electron microscopy (TEM) and X-ray photoemission spectroscopy (XPS) examined the island morphology and metal-insulator interactions, respectively. The effects of the island size, insulator vacancies, and band bending on the electrical transport of these GMs are discussed. How these factors change with post-growth annealing is also studied.    

Thermoelectric properties of four different ZnO nanostructures

In this work, we studied the thermoelectric properties of four different ZnO morphologies, namely, nanoribbons, nanorods, nanoparticles, and nanoshuttles. Experiments were performed from 324K to 364 K. Temperature-dependent Seebeck coefficients were observed by using Seebeck controller techniques. Electrical conductivity was measured by the Vander Pauw method. Seebeck coefficients for the four morphologies were negative which confirmed that all synthesized ZnO nanostructures are n-type semiconductors.  The absolute value of the Seebeck coefficients increased with increasing temperature. The electrical conductivity of ZnO morphologies also shows an increasing trend with temperature. Seebeck coefficients did not significantly change with different  ZnO morphologies. However, electrical conductivity showed a significant variation with ZnO morphology. The highest electrical conductivity (1080 Ω  ^-1m^-1) and electronic thermal conductivity (9.86 × 10^-3W/mK) were obtained in ZnO nanorods at 364K, whereas   ZnO nanoshuttles showed the lowest electrical conductivity (0.30 Ω  ^-1m^-1) and electronic thermal conductivity (1.48 × 10^-6 W/mK) at 324K. The maximum Power factor was obtained in ZnO nanorods at all temperatures compared to other morphologies. Our studies indicate that ZnO nanorods have the highest thermoelectric properties compared to other ZnO nano morphologies.     

Transition Metal Dichacogenides Film Growth using Metal Film Assisted Chemical Vapor Deposition

Atomically thin Transition metal dichacogenides (TMDCs) are potentially capable of miniaturizing optoelectronic devices to an atomic scale. However, this depends on the high-quality large area synthesis of materials. To address this need, we introduce metal film assisted chemical vapor deposition (MFACVD) for the wafer scale growth of TMDs. Recently, a successful wafer scale synthesis of high optical quality monolayer and few layer MoS₂ has been performed using this method and it shows the capability of synthesizing other TMDCs. Mainly we work on a new field of group IV TMDCs that have gained a considerable attention due to the prediction of excellent electronic properties. ZrSe₂ and HfSe₂ considered to have high in plane conductivity and higher mobility in MX₂ group and believed to be among the best candidates to replace Silicon in nano electronics due to its predicted band gap in the visible infrared region. Even though a considerable study has been done for the growth of bulk ZrSe₂ crystals, less study has been done for the large-scale synthesis of these films. In our method we deposit Zr films with different thickness on SiO₂ substrate and study the growth response using two different Se sources (Se powder and ZrSe₂ powders).   

Optical properties of poly(3-hexylthiophene) (P3HT) isolated chains and aggregates in solution: The role of chloroform stabilizer

With the improvement in synthetic methodologies, polythiophenes - specifically poly(3-hexylthiophene) (P3HT) - have become forerunners in the field of conjugated polymers. We study P3HT’s morphological and optical properties via measurements of absorption, photoluminescence (PL), and polarization-resolved Stokes spectropolarimetry. Our recent results show that P3HT’s aggregation behavior, in the precursor solution, changes drastically between using amylene-stabilized (Am-CHCl3) and non-stabilized chloroform (CHCl3). Particularly, P3HT solutions made from Am-CHCl3 presented H-aggregates, which is indicated by the higher PL intensity of 0-1 transition peak compared to 0-0 one. On the contrary, the similar samples diluted in CHCl3 exhibited J-aggregates with vibronic ratio  > 1. Our results indicate that the presence of amylene promotes the formation of -stacks between neighbor polymer chromophores and shortens conjugation length, leading to the formation of H-aggregates. To rule the influence of amylene out, we are currently performing a set of analyses and experiments consisting of various P3HT chain sizes and blended MeOH@CHCl3 solvents.   

Chemical vapor deposition of high-quality epitaxial bismuth ferrite films

High-quality thin films of room-temperature multiferroic BiFeO3 (BFO) have thus far been deposited primarily by pulsed laser deposition (PLD), a technique that cannot be scaled up readily. For device applications, it is essential to develop large-area thin film growth techniques such as chemical vapor deposition (CVD). We have used a direct liquid injection chemical vapor deposition (DLI-CVD) process to reproducibly grow high-quality epitaxial BFO films ranging in thickness from 20-200 nm, on SrTiO3 and DyScO3 substrates, using triphenyl bismuth(III) and Fe(III) acetylacetonate as metal-organic precursors. Smooth films having a root mean square (RMS) roughness ≤ 1nm are obtained that exhibit saturated ferroelectric hysteresis loops at room temperature for 200 nm thick films. BFO films that are slightly (2-4%) Bi-rich are found to have improved ferroelectric properties as compared to Fe-rich or stoichiometric films. The dependence of the morphological quality and ferroelectric properties on the stoichiometry and deposition parameters has been explored.  Ferroelectric phase switching of thinner films (20-80 nm) has been studied using piezo-response force microscopy (PFM) technique and the Kay-Dunn scaling law of switching field with film thickness is validated.   

In-situ TEM study of 2D-MoS2 FET

Defects are known to have a significant impact on the electrical properties of semiconducting two-dimensional (2D) transition metal dichalcogenides such as molybdenum disulfide (MoS₂). To understand this impact, it is crucial to establish a direct correlation between the electrical properties and local defect structures in MoS₂. Here, we have designed a MoS₂ field-effect transistor (FET) device that is compatible with high-resolution transmission electron microscopy (TEM) studies. In this MoS₂ FET device, graphite is used as electrodes and 2D hBN as the insulating substrate. We then transfer the stack onto a protochip TEM substrate for structural studies and in-situ electrical response measurement. We create defects such as triangular-shaped antidots in the MoS₂ FET device by annealing the MoS₂ in H₂/Ar environment. We then study how antidots with different edge structures affect the electrical properties of MoS₂.   

The sp2/sp3 bonding features of Fullerene-based Resistive Random Access Memory

The carbon-based materials, such as carbon nanotubes (CNTs), graphene, amorphous carbon (a-C) etc., have been addressed to the potential candidates for resistive random access memory(RRAM) devices. In this work, we studied the fullerene-based RRAMs with a simple structure of Au(50nm) / C₈₄ /Si (111). Ultrathin film of fullerene molecules (C₈₄) in our RRAM devices served as an insulating layer while Au(50nm) and high-doped Si(111) substrate worked as top and down electrodes.

The material characteristics, opto-electronic properties, ferromagnetic magnetism and the binding features of the fullerene-based RRAMs were performed with MFM, Raman, XPS, and PL. The formation and rupture of sp²/sp³ bonding switching between carbon atoms attributed to conductive filaments in RRAMs could be further investigated from various SPMs.  The resistive switching mechanism in our proposed fullerene-based RRAM is discussed.   

Crystalline character transition from amorphous to polycrystalline phase during La2O3 films growth using pulsed laser deposition

We have observed crystalline characteristics of high k dielectric La₂O₃ films transferring from amorphous to polycrystalline phase on Si substrates during the film growth using pulsed laser deposition method. During the film growth, reflection high-energy electron diffraction was used to monitor the growth phase transitions; it shows that initial crystalline patterns change to other at a certain time. In depth studies with transmission electron microscope exhibit clear formation of polycrystalline La₂O₃ layers on ~10 nm amorphous La₂O₃. Interestingly, no interfacial oxide layer between the Si substrate and the La₂O₃ film is formed. The amorphous phase turns out not from the interfacial mixing of Si and La₂O₃ layers. Rather, abrupt transition from amorphous to polycrystalline phase in La₂O₃ films is attributed to material’s growth nature.   

Obtaining Few Layers Tellurene Through Controlled Oxygen Annealing

Tellurene (Te) exhibits extraordinary properties such as tunable bandgap, anisotropic behavior, and resilience to ambient conditions, making it a desirable material for electronics and photoelectronic applications. However, exfoliation of a few layers of Te nanosheets can be challenging, leading to low yield. In this work, we show Te thinning using a thermal annealing technique applied on thick Te nanosheets. According to our AFM characterization, a considerable decrease in thickness was measured, with a decrease larger than 500nm. Raman spectroscopy was also used to identify the change in thickness of these thinned Te nanosheets. We observe a blue shift for A1 and E2 Raman modes after thinning, confirming the change in thickness of these nanosheets. Nevertheless, our method shows that thinning is non-uniform where a favorable thinning direction is observed for all Te nanosheets. This behavior can be attributed to the anisotropy behavior of Te, where thinning occurs along the major axis. Our results shed some light on an annealing method to thin Te nanosheets for future device applications.   

Relative stabilities of Si polytypes under the biaxial stress: A first principles study

Silicon is one of the most important elements for semiconductors and usually takes the diamond structure (3C). On the other hand, several experimental studies for synthesis of other polytypes of Si (2H [1] and 4H [2] phases) have been reported. The synthesis of the polytypes is of high importance and high interest due to the differences of electronic properties including the variation of band gap values and the effective mass values. In this work, we have studied the possibility of CVD homoepitaxial growth of the hexagonal Si polytypes on the 3H (3C) Si substrate under compression with biaxial stress. From the density functional theory (DFT), it is found that the 4H phase takes lower total energy than the 3H (3C) phase at the reduced lattice constant α. Hence, the 4H phase may grow on the (111) substrate under the biaxial stress. Furthermore, from the density functional perturbation theory (DFPT), it is found that there is possibility to synthesize not only the 4H phase but also the 2H phase at higher temperatures [3].

 

[1] R. H. Wentorf, Jr. and J. S. Kasper, Science 139, 338 (1963).

 [2] S. Pandolf et al., Nano Lett. 18, 5989 (2018).

 [3] R. Kita, M. Toyoda, and S. Saito, Phys. Rev. Mater. 5, 063402 (2021)   

Probing the trap levels in the wide bandgap TiO2 by Deep Level Transient Spectroscopy

Titanium dioxide (TiO₂) is an important material due to its application in various fields of science and technology, including medicine. However, it suffers from an enormous amount of native defects. Moreover, many of the exciting properties, such as resistive switching and intrinsic n-type conductivity, are governed by these. Hence, a genuine understanding of these defects will be essential and provide insight into designing the new devices based on TiO₂. In this, work we have fabricated a TiO₂ thin film-based Metal - Oxide - Semiconductor (MOS) capacitor to study the deep defects present in wide bandgap TiO₂ by Deep Level Transient Spectroscopy (DLTS) method. DLTS is a valuable tool for precisely studying the various defects parameters such as activation energy, capture-cross section, and density of traps. The analysis reveals five peaks in the DLTS spectrum, and in light of theoretical reports, we believe these belong to the oxygen vacancies and Ti interstitial related defects. These defects levels are located at 0.66 – 1.07 eV below the conduction band edge of TiO₂. The capture cross-sections and defect densities are in the range of 4.3×10^-17 – 3.2×10^-19 cm² and 1.9×10¹⁴ – 2.7×10¹⁶ cm^-3, respectively.   

Withdrawn

Thin films of SrNbO₃ perovskite has been emerged as a potential contender for substituting Tin doped Indium Oxide (ITO) for next generation transparent electrode material due to its low resistivity (~10^-5 Ω-cm) with high transparency (~65%) and carrier concentration (~10²² cm^-3). Moreover, the electrical conductivity can be made anisotropic by tuning its NbO₆ octahedra leading to pyroelectric, photoelectric, and multiferroic properties. Its native orthorhombic phase, on the other hand, is insulating in nature.  In this present work, we report on the growth of epitaxial SrNbO₃ (001) thin films on LaAlO₃ (001) or SrTiO₃ (001) substrates by pulsed laser deposition and show their optoelectronic properties along with those obtained from the DFT calculations. These films are characterized by high-resolution x-ray diffraction, UV-Visible spectroscopy, ellipsometry, AFM, XPS and electrical transport measurements. We first present the evidence of the existence of perovskite phase under suitable growth conditions and then show the manipulated electronic and optical properties for making it an attractive material for various kind of other optoelectronic applications.   

SrTi1-xTaxO3(x = 0-0.1) epitaxial thin films for next generation transparent electronics

Cubic SrTiO₃ is one of the most used indirect band gap (3.2 eV) oxide semiconductors. It displays many exotic properties, i.e., transparent conductivity, photocatalysis, ferroelectricity, two-dimensional electron gas, etc. due to the manipulations of defect chemistry and constituent elements via impurity doping.  Here, we report on the complexity of the optoelectronic and structural properties of the epitaxially stabilized 0-10 at.% Ta-doped SrTiO₃ (001) thin films on LaAlO₃ (001) substrates by selectively  varying the growth temperature (650-850 °C) and oxygen partial pressure (5×10^-2 - 5×10^-7 mbar) during the pulsed laser deposition using a 248 nm excimer laser with an energy density of 1.5 J/cm² at the repetition rate of 5 Hz.  These films are characterized by high-resolution XRD, RBS-channelling, AFM, UV, HR-XPS and electrical measurements. All the films are found to be epitaxial in nature, stoichiometric composition, excellent uniformity in thickness and transparent in the UV region. The effect of growth parameters and Ta dopant on the epitaxial quality of these layers are understood by determining the dopant location and its concentration in the SrTiO₃ lattice. The expansion of the lattice parameter of SrTiO₃ with Ta concentration is explained by computational modelling. The complex relationships of optical and electronic properties on growth parameters, dopant concentration, and single crystal quality of the films are also found. Metallic properties in the film enhance with decreasing pO₂ and increasing growth temperature. The obtained room-temperature resistivity, carrier density, and mobility from the optimized sample turn out to be ∼5×10⁻³ Ω cm, 2.63×10²⁰/cm³, and 4.5 cm² /V s, respectively and high optical transparency ∼85%–90%, offer it as an exciting material for next generation transparent optoelectronics.