Within its rich phase diagram titanium dioxide is a truly multifunctional material with a property palette that has been shown to span from dielectric to transparent-conducting characteristics, in addition to the well-known catalytic properties. At the same time down-scaling of microelectronic devices has led to an explosive growth in research on atomic layer deposition (ALD) of a wide variety of frontier thin-film materials, among which TiO2 is one of the most popular ones. In this topical review we summarize the advances in research of ALD of titanium dioxide starting from the chemistries of the over 50 different deposition routes developed for TiO2 and the resultant structural characteristics of the films. We then continue with the doped ALD-TiO2 thin films from the perspective of dielectric, transparent-conductor and photocatalytic applications. Moreover, in order to cov er the lat est tr ends in t he research f ield, both the var iously constr ucted TiO2 nanostructures enabled by ALD and the Ti-based hybrid inorganic-organic films grown by the emerging ALD/MLD (combined atomic/molecular layer deposition) technique are discussed.

Nanomaterial interfaces and concomitant thermal resistances are generally considered as atomic-scale planes that scatter the fundamental energy carriers. Given that the nanoscale structural and chemical properties of solid interfaces can strongly influence this thermal boundary conductance, the ballistic and diffusive nature of phonon transport along with the corresponding phonon wavelengths can affect how energy is scattered and transmitted across an interfacial region between two materials. In hybrid composites composed of atomic layer building blocks of inorganic and organic constituents, the varying interaction between the phononic spectrum in the inorganic crystals and vibronic modes in the molecular films can provide a new avenue to manipulate the energy exchange between the fundamental vibrational energy carriers across interfaces. Here, we systematically study the heat transfer mechanisms in hybrid superlattices of atomic-and molecular-layer-grown zinc oxide and hydroquinone with varying thicknesses of the inorganic and organic layers in the superlattices. We demonstrate ballistic energy transfer of phonons in the zinc oxide that is limited by scattering at the zinc oxide/hydroquinone interface for superlattices with a single monolayer of hydroquinone separating the thicker inorganic layers. The concomitant thermal boundary conductance across the zinc oxide interfacial region approaches the maximal thermal boundary conductance of a zinc oxide phonon flux, indicative of the contribution of long wavelength vibrations across the aromatic molecular monolayers in transmitting energy across the interface. This transmission of energy across the molecular interface decreases considerably as the thickness of the organic layers are increased.

TiO2:C superlattices are fabricated from atomic/molecular layer deposited (ALD/MLD) inorganic-organic [(TiO2)m(Ti-O-C6H4-O-)k=1]n thin films via a post-deposition annealing treatment that converts the as-deposited monomolecular organic layers into sub-nanometerthick graphitic interface layers confined within the TiO2 matrix. The internal graphitic layers act as effective phonon-scattering boundaries that bring about a ten-fold reduction in thermal conductivity of the films with decreasing superlattice period down to an ultra-low value of 0.66±0.04 Wm -1 K -1 -a finding that makes inorganic-C superlattices fabricated with the present method as promising structures for e.g. high-temperature thermal barriers and thermoelectrics. IntroductionMaterials with ultra-low thermal conductivity are needed, e.g., for thermal-barrier and thermoelectric applications; the latter application calls for novel heavily-doped semiconductors able to combine thermal insulation with high electronic conductivity and thermopower. General pathways to suppress thermal transport in fully-dense solid materials exploit the introduction of structural disorder in the form of, e.g., point defects, alloying components, amorphous phases, grain boundaries or material interfaces. Both experiments and theory have shown that the introduction of material interfaces is particularly well harnessed in various superlattice and multilayer thin-film materials where the interfaces between alternating layers of dissimilar materials act as phonon-scattering boundaries: not only may thermal conductivity be suppressed by an order of magnitude across the film plane 1-2 but a significant drop may also be seen in the inplane direction. [3][4] Furthermore, careful balancing between order and disorder in multilayers may enable achieving ultralow thermal conductivities comparable to or even lower than those of amorphous or porous materials, as evidenced by the results for, e.g., W/Al2O3 nanolaminates (~0.6 Wm -1 K -1 ) and layered WSe2 crystals (~0.05 Wm -1 K -1 ). [5][6][7] For small-period superlattices the dominance of the phononboundary scattering at the internal interfaces over the scattering by the bulk of the constituent materials enables the control of thermal conductivity through careful adjustment of the superlattice period; 8 efficient suppression is achieved for incoherent phonons by decreasing the period, until potentially, phonon coherence may yield an upturn for periods similar to (and smaller than) phonon mean free path. 9-10 However, control over thermal conductivity in layered materials is not limited to simple size effects, as in particular for inorganic-organic materials, the drastic mismatch of the vibrational properties and the control over the bond strength over the internal interfaces may allow for further suppression of phonon transport. [11][12] Regarding inorganic-organic materials, use of organic layers of proper thicknesses could also allow for exploitation of phonon filtering realized due to interference effects within the organic layer. 13...

Nanoscale layer-engineering is an attractive tool to tailor the performance of thermoelectric materials as it potentially allows us to suppress thermal conductivity without significantly hindering the electrical transport properties. By combining the state-of-the-art thin-film fabrication technique for inorganics, i.e. atomic layer deposition (ALD), with its emerging counterpart for the organics, i.e. molecular layer deposition (MLD), it is possible to fabricate in a single reactor oxideorganic thin-film superlattices in which periodically introduced single/few-molecule organic layers alternate with thicker thermoelectric oxide layers. In such fundamentally new types of superlattice materials the oxide-organic interfaces with notable property mismatch are anticipated to hinder the phonon transport and/or bring about charge confinement effects thereby enhancing the material´s thermoelectric figure-of-merit. The experimental data so far gathered for the (Zn,Al)O:HQ and (Ti,Nb)O2:HQ systems (HQ stands for hydroquinone) show significantly suppressed thermal conductivities. Here in this topical review we summarize the experimental and computational works carried out on these superlattice materials and discuss the future potential of the ALD/MLD-fabricated inorganic-organic superlattice and nanolaminate thin-film structures in thermoelectrics.

The incorporation of organic layers was found to systematically blue-shift the optical band gap of TiO2 with decreasing superlattice period, and -most importantly -to sensitize the TiO2 layers to visible light over a considerable part of the visible range below 700 nm, a fact that could be of substantial interest in photocatalysis and solar cell applications.

Pliable and lightweight thin-film magnets performing at room temperature are indispensable ingredients of the next-generation flexible electronics. However, conventional inorganic magnets based on f-block metals are rigid and heavy, whereas the emerging organic/molecular magnets are inferior regarding their magnetic characteristics. Here we fuse the best features of the two worlds, by tailoring ε-Fe 2 O 3 -terephthalate superlattice thin films with inbuilt flexibility due to the thin organic layers intimately embedded within the ferrimagnetic ε-Fe 2 O 3 matrix; these films are also sustainable as they do not contain rare heavy metals. The films are grown with sub-nanometer-scale accuracy from gaseous precursors using the atomic/molecular layer deposition (ALD/MLD) technique. Tensile tests confirm the expected increased flexibility with increasing organic content reaching a 3-fold decrease in critical bending radius (2.4 ± 0.3 mm) as compared to ε-Fe 2 O 3 thin film (7.7 ± 0.3 mm). Most remarkably, these hybrid ε-Fe 2 O 3 -terephthalate films do not compromise the exceptional intrinsic magnetic characteristics of the ε-Fe 2 O 3 phase, in particular the ultrahigh coercive force (∼2 kOe) even at room temperature.

Atomic layer deposition (ALD) is a vital gas-phase technique for atomic-level thickness-controlled deposition of high-quality thin films on various substrate morphologies owing to its self-limiting gas-surface reaction mechanism. Here we report the ALD fabrication of thin films of the semiconducting CuCrO2 oxide that is a highly prospective candidate for transparent electronics applications. In our process, copper 2,2,6,6-tetramethyl-3,5-heptanedionate (Cu(thd)2) and chromium acetyl acetonate (Cr(acac)3) are used as the metal precursors and ozone as the oxygen source. Smooth and homogeneous thin films with an accurately controlled metal composition can be deposited in the temperature range of 240-270 o C; a post-deposition anneal at 700-950 o C i n a n A r atmosphere then results in well crystalline films with the delafossite structure. Electrical transport measurements confirm the p-type semiconducting behavior of the films. The direct bandgap is determined f r o m U V -v i s s p e c t r o p h o t o m e t r i c m e a s u r e m e n t s t o b e 3 . 0 9 e V . T h e observed transmittance is greater than 75% in the visible range.

We characterized transport and optical properties of atomic layer deposited Nb:TiO2 thin films on glass substrates. These promising transparent conducting oxide (TCO) materials show minimum resistivity of 1.0 × 10−3 Ω cm at 300 K and high transmittance in the visible range. Low-temperature (2–300 K) Hall measurements and the Drude fitting of the Vis-NIR optical spectra indicate a transition in the scattering mechanism from grain boundary scattering to intra-grain scattering with increasing Nb content, thus underlining enhancement of the grain size in the low doping regime as the key for further improved TCO properties.