Multilayered ZnO-based thin films to control heat and electrical transport properties

Defense is held on 26.3.2021 12:00 – 15:00 https://aalto.zoom.us/j/62499185588Interfaces between materials can have properties that differ greatly from the bulk state. In classical materials only a tiny fraction of atoms are at the interface while the vast majority is in the bulk of the material. The capability to engineer materials with an artificially high amount of interfaces opens up a pathway to amplify the interface effects and tailor the material properties by controlling the amount of interfaces. This approach to engineer materials step by step or layer by layer also allows for a controlled combination of very different materials into a hybrid material that would not form naturally and which can show fundamentally different and new properties. In this thesis atomic layer deposition (ALD), molecular layer deposition (MLD) and pulsed laser deposition (PLD) are utilized to engineer ZnO-based thin films with high interface densities. The films are analysed with x-ray reflectivity (XRR), x-ray diffraction (XRD) and transmission electron microscopy (TEM) in regards to their internal structure. Time domain thermoreflectance (TDTR) is utilized to measure the thermal conductivity, the electrical properties are measured with a hall measurement setup. The latter is the focus in layered thin films of polycrystalline ZnO and amorphous InGaZnO4 in which a considerable increase in the charge carrier concentration following the interface density could be demonstrated. The interfaces between a ZnO matrix, ZnO-benzene and AlOx layers are studied in detail in a hybrid ZnO/ZnO-benzene/AlOx system in which this work demonstrates, that these layers in ZnO can be as thin as a single atom/molecule, yet still form distinctive layers. However, these very thin layers of ZnO-benzene and AlOx are found to have little impact on the crystal growth of ZnO, but can act as effective barriers for ZnO crystal growth when 10 or more consecutive ALD/MLD cycles are utilized for each AlOx/benzene layer respectively. Finally the thermal conductivity in ZnO/benzene thin films is characterised, the database for the thermal conductivity in that system is significantly extended and thermal conductivities for irregularly layered structures are reported for the first time in ZnO/ZnO-benzene hybrid thin films. Analysis with multivariate data analysis of the database confirms that the interface density has the most pronounced effect on the thermal conductivity.

Spontaneous Generation of Carrier Electrons at the Interface between Polycrystalline ZnO and Amorphous InGaZnO4

The interface between two materials can be expected to show exotic optical, electrical, and thermal transport properties due to the difference in chemical bonding and chemical potential. However, in conventional material systems, the volume fraction of the interface is small compared to bulk, and interfacial properties are thus difficult to utilize. In this regard, multilayered films are essential to increase the volume fraction of interfaces and functionalize their properties. Here it is shown that carrier electrons can be generated spontaneously at the interface between polycrystalline ZnO and amorphous (a-) InGaZnO4. The electron transport properties are measured of multilayered films composed ofc-axis oriented polycrystalline ZnO and a-InGaZnO(4)with varying interface density (d(-1)). Although the carrier concentrations of both ZnO and a-InGaZnO(4)are less than 5 x 10(19)cm(-3), thenincreases withd(-1)and exceedes 10(20)cm(-3). The relatively large interface thermal resistance between ZnO and a-InGaZnO4(1.35 m(2)K GW(-1)) indicates the existence of a large difference in the chemical bonding and the chemical potential and thus conduction electrons would accumulate at the interface

Characterization of ZnO/AlO x/benzene thin-film heterostructures grown through atomic layer deposition/molecular layer deposition

| openaire: EC/H2020/339478/EU//LAYERENG-HYBMATMultilayer thin-film structures are promising for many future high-tech applications. We investigate the structure of polycrystalline ZnO thin films with sub-nanometer amorphous inorganic (AlO x ) and organic (benzene) layers grown by atomic/molecular layer deposition. Small quantities of aluminium are typically introduced in ZnO films for doping, while one of the intended functions of the organic layers is to block thermal conductivity. We apply the AlO x and benzene layers both simultaneously and separately, and investigate the resultant superlattice films with transmission electron microscopy, x-ray reflectivity and x-ray diffraction measurements. The study reveals that both AlO x and benzene form distinct layers in the ZnO matrix even down to one atomic/molecular layer. Furthermore, we demonstrate that despite the clear layering, the ZnO grains can penetrate through thin (below ca. 2 nm) benzene and AlO x layers.Peer reviewe

Toward Luminescent Composites by Phase Transfer of SrF2:Eu3+ Nanoparticles Capped with Hydrophobic Antenna Ligands

Transparent dispersions of hydrophobic SrF2 : Eu3+ nanoparticles in cyclohexane with up to 20% europium were obtained by fluorolytic sol‐gel synthesis followed by phase transfer into cyclohexane through capping with sodium dodecylbenzenesulfonate (SDBS). The particles were characterized by TEM, XRD and DLS as spherical objects with a diameter between 6 and 11 nm in dry state. 1H‐13CP MAS NMR experiments revealed the binding of the anionic sulfonate head group to the particle surface. The particles show bright red luminescence upon excitation of the aromatic capping agents, acting as antennas for an energy transfer from the benzenesulfonate unit to the Eu3+ centers in the particles. This synthesis method overcomes the current obstacle of the fluorolytic sol‐gel synthesis that transparent dispersions can be obtained directly only in hydrophilic solvents. To demonstrate the potential of such hydrophobized alkaline‐earth fluoride particles, transparent luminescent organic‐inorganic composites with 10% SrF2 : Eu3+ embedded into polyTEGDMA, polyBMA, polyBDDMA and polyD3MA, respectively, were prepared, endowing the polymers with the luminescence features of the nanoparticles.Peer Reviewe

Spontaneous Generation of Carrier Electrons at the Interface between Polycrystalline ZnO and Amorphous InGaZnO 4

The interface between two materials can be expected to show exotic optical, electrical, and thermal transport properties due to the difference in chemical bonding and chemical potential. However, in conventional material systems, the volume fraction of the interface is small compared to bulk, and interfacial properties are thus difficult to utilize. In this regard, multilayered films are essential to increase the volume fraction of interfaces and functionalize their properties. Here it is shown that carrier electrons can be generated spontaneously at the interface between polycrystalline ZnO and amorphous (a-) InGaZnO4. The electron transport properties are measured of multilayered films composed ofc-axis oriented polycrystalline ZnO and a-InGaZnO(4)with varying interface density (d(-1)). Although the carrier concentrations of both ZnO and a-InGaZnO(4)are less than 5 x 10(19)cm(-3), thenincreases withd(-1)and exceedes 10(20)cm(-3). The relatively large interface thermal resistance between ZnO and a-InGaZnO4(1.35 m(2)K GW(-1)) indicates the existence of a large difference in the chemical bonding and the chemical potential and thus conduction electrons would accumulate at the interface

Thermal Conductivity Reduction at Inorganic-Organic Interfaces

| openaire: EC/H2020/339478/EU//LAYERENG-HYBMATNanoscale superlattice structures are known to significantly suppress the thermal conductivity in thin films due to phonon scattering at the interfaces of the mutually different layers. Here it is demonstrated that in addition to the number of interfaces, their spacing within the film can lead to a reduction in thermal conductivity. The proof-of-concept data are for ZnO/benzene thin films fabricated through sequential gas-surface reactions in atomic/molecular layer precision using the atomic/molecular layer deposition technique. In comparison to similarly constructed regular superlattice thin films, thermal conductivity values that are of the same magnitude, or even lower, are achieved for hybrid ZnO/benzene thin films in which the inorganic and organic layers are arranged in a more irregular manner to form various gradient patterns.Peer reviewe

Experimental Control and Statistical Analysis of Thermal Conductivity in ZnO-Benzene Multilayer Thin Films

| openaire: EC/FP7/339478/EU//LAYERENG-HYBMATWe have fabricated a model system of precisely layer-engineered inorganic-organic thin-film structures using atomic/molecular-layer deposition (ALD/MLD). The samples consist of nanoscale polycrystalline ZnO layers and intervening benzene layers, covering a broad range of layer sequences. The samples characterized in this study combined with previous publications provide an excellent sample set to examine thermal transport properties in inorganic-organic thin films. The cross-plane thermal conductivity is found to depend on multiple factors, with the inorganic-organic interface density being the dominating factor. Our work highlights the remarkable capability of interface engineering in suppressing the thermal conductivity of hybrid inorganic-organic materials, e.g., for thermoelectric applications.Peer reviewe

Low-Temperature Molecular Layer Deposition Using Monofunctional Aromatic Precursors and Ozone-Based Ring-Opening Reactions

Molecular layer deposition (MLD) is an increasingly used deposition technique for producing thin coatings consisting of purely organic or hybrid inorganic-organic materials. When organic materials are prepared, low deposition temperatures are often required to avoid decomposition, thus causing problems with low vapor pressure precursors. Monofunctional compounds have higher vapor pressures than traditional bi- or trifunctional MLD precursors, but do not offer the required functional groups for continuing the MLD growth in subsequent deposition cycles. In this study, we have used high vapor pressure monofunctional aromatic precursors in combination with ozone-triggered ring-opening reactions to achieve sustained sequential growth. MLD depositions were carried out by using three different aromatic precursors in an ABC sequence, namely with TMA + phenol + O3, TMA + 3-(trifluoromethyl)phenol + O3, and TMA + 2-fluoro-4-(trifluoromethyl)benzaldehyde + O3. Furthermore, the effect of hydrogen peroxide as a fourth step was evaluated for all studied processes resulting in a four-precursor ABCD sequence. According to the characterization results by ellipsometry, infrared spectroscopy, and X-ray reflectivity, self-limiting MLD processes could be obtained between 75 and 150 °C with each of the three aromatic precursors. In all cases, the GPC (growth per cycle) decreased with increasing temperature. In situ infrared spectroscopy indicated that ring-opening reactions occurred in each ABC sequence. Compositional analysis using time-of-flight elastic recoil detection indicated that fluorine could be incorporated into the film when 3-(trifluoromethyl)phenol and 2-fluoro-4-(trifluoromethyl)benzaldehyde were used as precursors.Peer reviewe