Extreme nanowires (ENs) represent the ultimate class of crystals: They are the smallest possible periodic materials. With atom-wide motifs repeated in one dimension (1D), they offer a privileged perspective into the physics and chemistry of low-dimensional systems. Single-walled carbon nanotubes (SWCNTs) provide ideal environments for the creation of such materials. Here we present a comprehensive study of Te ENs encapsulated inside ultranarrow SWCNTs with diameters between 0.7 nm and 1.1 nm. We combine state-of-the-art imaging techniques and 1D-adapted ab initio structure prediction to treat both confinement and periodicity effects. The studied Te ENs adopt a variety of structures, exhibiting a true 1D realization of a Peierls structural distortion and transition from metallic to insulating behavior as a function of encapsulating diameter. We analyze the mechanical stability of the encapsulated ENs and show that nanoconfinement is not only a useful means to produce ENs but also may actually be necessary, in some cases, to prevent them from disintegrating. The ability to control functional properties of these ENs with confinement has numerous applications in future device technologies, and we anticipate that our study will set the basic paradigm to be adopted in the characterization and understanding of such systems.

Nanostructuring, e. g., reduction of dimensionality in materials, offers a viable route toward regulation of materials electronic and hence functional properties. Here, we present the extreme case of nanostructuring, exploiting the capillarity of single-walled carbon nanotubes (SWCNTs) for the synthesis of the smallest possible SnTe nanowires with cross sections as thin as a single atom column. We demonstrate that by choosing the appropriate diameter of a template SWCNT, we can manipulate the structure of the quasi-one-dimensional (1D) SnTe to design electronic behavior. From first principles, we predict the structural re-formations that SnTe undergoes in varying encapsulations and confront the prediction with TEM imagery. To further illustrate the control of physical properties by nanostructuring, we study the evolution of transport properties in a homologous series of models of synthesized and isolated SnTe nanowires varying only in morphology and atomic layer thickness. This extreme scaling is predicted to significantly enhance thermoelectric performance of SnTe, offering a prospect for further experimental studies and future applications.

One-dimensional (1D) atomic chains of CsI were previously reported in double-walled carbon nanotubes with ∼0.8 nm inner diameter. Here, we demonstrate that, while 1D CsI chains form within narrow ∼0.73 nm diameter single-walled carbon nanotubes (SWCNTs), wider SWCNT tubules (∼0.8−1.1 nm) promote the formation of helical chains of CsI 2 × 1 atoms in cross-section. These CsI helices create complementary oval distortions in encapsulating SWCNTs with highly strained helices formed from strained Cs 2 I 2 parallelogram units in narrow tubes to lower strain Cs 2 I 2 units in wider tubes. The observed structural changes and charge distribution were analyzed by density-functional theory and Bader analysis. CsI chains also produce conformation-selective changes to the electronic structure and optical properties of the encapsulating tubules. The observed defects are an interesting variation from defects commonly observed in alkali halides as these are normally associated with the Schottky and Frenkel type. The energetics of CsI 2 × 1 helix formation in SWCNTs suggests how these could be controllably formed.

The electrical conductivity of metallic carbon nanotubes (CNTs) quickly saturates with respect to bias voltage due to scattering from a large population of optical phonons. Decay of these dominant scatterers in pristine CNTs is too slow to offset an increased generation rate at high voltage bias. We demonstrate from first principles that encapsulation of 1D atomic chains within a single-walled CNT can enhance decay of "hot" phonons by providing additional channels for thermalisation. Pacification of the phonon population growth reduces electrical resistivity of metallic CNTs by 51% for an example system with encapsulated beryllium.Carbon Nanotubes (CNTs) are the most promising candidates for nanoelectronics applications due to their excellent electrical conductivity [1][2][3]. With these properties applied in integrated circuits and in the field effect transistors' gate electrodes metallic CNTs lead the minituarisation race at the nanoscale [4]. However experimental and theoretical studies show that electronic transport in metallic CNTs undergoes a dramatic decrease beyond a bias of ≈ 0.2 eV due to scattering of conduction electrons from a population of high frequency phonons [5][6][7][8]. Under bias voltage, the process of electron scattering excites new phonons into this population an order of magnitude faster than their decay via thermalisation [9]. This dominance of excitation over deexcitation results in a growing population of athermal "hot" phonons and consequently a non-equilibrium phonon distribution [10,11]. Under such conditions, these hot phonons constitute the dominant source of electron scattering, and hence resistivity, in metallic CNTs. A mechanism for increasing the rate of phonon thermalisation will therefore enhance the electrical performance of CNTs.The process of phonon deexcitation involves anharmonic phonon-phonon scattering, hence its rate is dependent on the number of available channels for phonon decay. Previously, several possible solutions for introduction of additional thermalisation channels were suggested that considered a supporting substrate or isotopic disorder [12][13][14].In this letter we show that reduction of the hot phonon population under bias is readily achievable via encapsulation of 1D nanowires in single-walled CNTs (SWCNT). We consider phonon-phonon relaxation from first principles and demonstrate that an encapsulated one-dimensional crystal creates additional channels for hot phonon thermalisation, increasing the decay rate. This results in a significant improvement of the voltage-current ratio at high bias voltage. Transport is studied by solving the set of parameter-free Boltzmann transport equations (BTE) for coupled dynamics of electrons and phonons, whilst all relevant scattering rates are calculated ab initio with densityfunctional perturbational theory (DFPT). This route to enhanced transport is attractive due to increasingly well-established methods for growth of 1D crystals inside CNTs [15][16][17][18][19] and assembly of nanowires into integrated devices [...

Germanium telluride has attracted great research interest, primarily because of its phase-change properties. We have developed a general scheme, based on the ab initio random structure searching (AIRSS) method, for predicting the structures of encapsulated nanowires, and using this we predict a number of thermodynamically stable structures of GeTe nanowires encapsulated inside carbon nanotubes of radii under 9Å. We construct the phase diagram of encapsulated GeTe, which provides quantitative predictions about the energetic favorability of different filling structures as a function of the nanotube radius, such as the formation of a quasi-one-dimensional rock-salt-like phase inside nanotubes of radii between 5.4 and 7.9Å. Simulated TEM images of our structures show excellent agreement between our results and experimental TEM imagery. We show that, for some nanotubes, the nanowires undergo temperature-induced phase transitions from one crystalline structure to another due to vibrational contributions to the free energy, which is a first step toward nano-phase-change memory devices.