The relative orientation of successive sheets, i.e. the stacking sequence, in layered two-dimensional materials is central to the electronic, thermal, and mechanical properties of the material. Often different stacking sequences have comparable cohesive energy, leading to alternative stable crystal structures. Here we theoretically and experimentally explore different stacking sequences in the van der Waals bonded material hexagonal boron nitride (h-BN). We examine the total energy, electronic bandgap, and dielectric response tensor for five distinct high symmetry stacking sequences for both bulk and bilayer forms of h-BN. Two sequences, the generally assumed AA' sequence and the relatively unknown (for h-BN) AB (Bernal) sequence, are predicted to have comparably low energy. We present a scalable modified chemical vapor deposition method that produces large flakes of virtually pure AB stacked h-BN; this new material complements the generally available AA' stacked h-BN.In recent years there has been a dramatic resurgence in interest in van der Waals bonded layered materials, including graphite, boron-nitride, and transition metal dichalcogenides. 1,2 These materials display strong intraplane (typically covalent) bonding and weak van der Waals interplane bonding, which facilitates exfoliation into mono-layer or few-layer forms, and further allows custom stack-ups or laminations of sheets with different chemical composition or crystallographic orientation. 1-8 Even for a material composed of identical sheets, the stacking order of the successive sheets or layers, which may be translationally and/or rotationally shifted, can profoundly influence the overall physical properties. 9-12 For example, for naturally occurring graphite the usual stacking sequence is Bernal (AB) stacking, but rhombohedral (ABC) stacking is also possible, which has a completely different electronic band structure. 13,14 Hexagonal boron nitride (h-BN) is structurally very similar to graphite, with successively stacked (and van der Waals bonded) sheets of hexagonally arranged sp 2bonded boron and nitrogen. 5 However, unlike graphite, h-BN is purely synthetic, with a wide electronic band gap (hence the nickname "white graphite"). [15][16][17] Virtually all synthesis methods for h-BN lead to an AA' stacking sequence, where atoms in one layer all lie directly above atoms in the next layer. 4,[18][19][20] Successive layers are rotated such that all nitrogens lie above borons, and all borons lie above nitrogens. 20 Potential alternative stacking sequences for h-BN are of great theoretical and experimental interest.Here we explore these alternative h-BN stacking sequences. We employ Density Functional Theory (DFT) to determine the total energy, electronic band structure, and dielectric tensor elements for five different high-symmetry h-BN stackings. We find that Bernal (AB) stacked h-BN has a total energy comparable to, and indeed a bit lower than,

We present a comprehensive first principles study of doped hafnia in order to understand the formation of the ferroelectric orthorhombic [001] grains. Assuming that tetragonal grains are present during the early stages of growth, matching plane analysis shows that tetragonal [100] grains can transform into orthorhombic [001] during thermal annealing, when they are laterally confined by other grains. We show that among 0%, 2% and %4 Si doping, 4% doping provides the best conditions for the tetragonal [100] → orthorhombic [001] transformation. This also holds for Al doping. We also show that for HfxZr1−xO2, where we have studied x = 1.00, 0.75, 0.50, 0.25, 0.00, the value x = 0.50 provides the most favorable conditions for the desired transformation. In order for this transformation to be preferred over the tetragonal [100] → monoclinic [100] transformation, out-ofplane confinement also needs to be present, as supplied by a top electrode. Our findings illuminate the mechanism that causes ferroelectricity in hafnia-based films and provide an explanation for common experimental observations for the optimal ranges of doping in Si:HfO2, Al:HfO2 and HfxZr1−xO2. We also present model thin film heterostructure computations of Ir/HfO2/Ir stacks in order to isolate the interface effects, which we show to be significant. ferroelectricity in HfO 2 thin films"