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Over the last 15 years, the content of Nature Physics has covered an enormous breadth of subjects at the forefront of physics. The journal’s past and present editors recount their favourite papers and what made chaperoning them to publication special.

Slotting a fusion reactor into the heart of a nuclear fission plant could accelerate the development of waste-free nuclear energy. So why are all the designs still on paper, asks Ed Gerstner.

The 2015 Nature Awards for Mentoring in Science recognize Chinese scientists who have invested in the next generation.

Chromium disilicide layers have been formed by ion bombardment through a chromium layer into silicon at room temperature. It was found that the thickness of the silicide formed under the chromium layer was similar for hydrogenated amorphous silicon, laser crystallized silicon, and crystalline silicon substrates. The growth was consistent with a radiation enhanced diffusion mechanism in which the rate limiting process was the diffusion of silicon or chromium through the silicide. It was found that the amount of chromium in the silicide and its thickness were proportional to the square root of the total amount of energy dissipated by the ion beam at the interface between the chromium and the substrate.

This paper reports physical and psychophysical investigations of the discriminability of Australian banknotes, prior to release of the current $5 note in 1995, and proposes methods for assessing distinctiveness within any set of coloured banknotes. Average note colour was measured under the CIE standard illuminants A, B, C, D65 and F2, and the positions of the notes in three-dimensional CIELAB colour space were plotted as a means of determining relative closeness by way of the A-difference measure of proximity in colour space. Building on hypotheses derived from the physical measurements, four psychophysical experiments were carried out. Experiment 1 required participants to judge sameness and difference of computer-presented images of notes while the dimensions of colour, design and size were manipulated. In Experiment 2, participants sorted notes under two levels of illumination and the dimensions of colour and design were again manipulated. Experiment 3 required participants to decide whether a computer-presented image of a note was one designated by the experimenter, and Experiment 4 had participants detecting target notes in computer-presented, simulated wallets. Results supported the release of the new re-coloured $5 note and demonstrated its superior discriminability over that of the old and now discontinued $5 note. Results are also discussed in the context of ongoing theoretical issues in multidimensional stimulus discrimination.

Modification of tetrahedral amorphous carbon by nitrogen addition or by heating has important effects on its electrical conductivity. In the case of nitrogen addition at a level of approximately 3% a new conductivity mode is introduced which can be turned off by electrical bias. This mode can be used to form a memory device. Using first principles molecular dynamics simulations in the local density approximation and local spin density approximation, we study the structural changes resulting when an electron is removed from the nitrogen atom. Heating also changes the electrical conductivity in a permanent way, producing a thermodynamically stable form of carbon with a high degree of sp2 bonding. We show that this effect can be used in producing permanent connections between two metal layers in a structure known as an antifuse, and that tetrahedral amorphous carbon is far superior in this application to other choices such as silicon nitride, aluminium nitride and amorphous silicon.

Recently, we reported on reversible non-volatile memory effects in nitrogen doped tetrahedral amorphous carbon (ta-C:N) thin films, and the properties of 1-bit memory cells fabricated to utilize such effects. In this paper, we investigate the electrical stability of ta-C:N devices with respect to the effects of many repetitive read, write, and erase cycles on their ON and OFF state resistances.

The introduction of nitrogen dopant sites into tetrahedral amorphous carbon produces changes in the structure and the electronic density of states that can be modelled using molecular dynamics. In this work we use both a tight-binding approach and a Car-Parrinello density functional theory approach. In a comparison of these, we found that the former tends to overestimate the strain energy of 3 membered carbon rings relative to the latter and to experiment, explaining the reduced occurrence of 3 membered rings in networks and simulated using tight-binding. Experiment shows that at approximately 3% of nitrogen, the network begins to change rapidly with nitrogen content. In this form, an additional electronic conduction mode is found experimentally, of the Poole-Frenkel type, which can be turned on and off at will. The conduction is turned on by negative voltage excursion and quenched by a positive one. This conduction bistability can be exploited to produced a simple new type of memory device in which the high conductivity state (''on'') is a digital ''1'' and the low conductivity state (''off'') is a digital ''0''. The operating characteristics of the device are excellent, with more than one million read cycles having been demonstrated without deterioration of the discrimination between the ''on'' and ''off'' states. Molecular dynamics is used to study the configuration of the nitrogen atoms, yielding a possible candidate for the site responsible for the Poole-Frenkel conduction.

It has previously been shown that ion beams may be used to provide useful modifications to the properties of polymers. For example, it has been found that hydrogen and argon ion beams provide large increases in the hardness of the polymer CR39, a polycarbonate used extensively in optical applications. In this paper we report the structural effects of the ion-beam modification of CR39, determined using Fourier transform infrared reflectance spectroscopy, cross-sectional electron microscopy, energy-dispersive X-ray spectroscopy and electron-energy-loss spectroscopy. Such investigations show densification, the development of compressive stress, the evolution of oxygen and hydrogen, and an increase in C = C bonding presumably as a result of cross-linking in the polymer chains.

Physicists are planning lasers powerful enough to rip apart the fabric of space and time. Ed Gerstner is impressed.

Electrical measurements of nitrogen doped tetrahedral amorphous carbon (ta-C:N) thin films have revealed a reversible nonvolatile memory effect, related to the excitation and de-excitation of electrons between deep acceptor states and shallow donor states within the mobility gap. This effect is characterized by changes in the small signal film conductivity of up to 10 times, and has been used to fabricate 1-bit memory cells with effective memory retention times in the order of several months.

The response of tetrahedral amorphous carbon to irradiation with 200 keV Xe + ion irradiation at various temperatures is reported. The structure of the tetrahedral amorphous carbon following ion irradiation is monitored in-situ by electrical resistance measurements, and ex-situ by Raman spectroscopy, electron energy loss spectroscopy and electron diffraction. It is found that the irradiation temperature strongly influences the final state of the ion irradiated tetrahedral amorphous carbon. For irradiation at room temperature and below, the ion beam transforms the material into a conducting amorphous carbon with an sp 2 fraction of about 0.6. The effect of elevated irradiation temperatures is to increase the sp 2 fraction to about 0.8 at the highest doses and to increase the degree of ordering. The ordering is distinctive in that there is a high degree of in-plane disorder combined with a regular stacking of the sp 2 bonded planes. The effect of temperature on the ion beam induced transformation of tetrahedral amorphous carbon resembles that for the ion beam transformation of diamond for which low temperature irradiations produces a damaged amorphous structure, whereas for high temperatures a graphitic material is produced.

Filtering of plasmas by curved solenoidal ducts is well established as a method of removing macroparticles. By analyzing the interactions of planar probes with the drifting plasma of the cathodic arc, new insights have been obtained into the operation of these ducts. Theoretical modeling of these interactions suggests, and experiment confirms, that the use of a separate biased electrode on the inside of the duct gives enhanced transmission without drawing excessive electron current. Theoretical modeling of a negatively biased planar electrode lying parallel to the drift velocity as well as experiment both show that ions are captured effectively onto the electrode producing a macroparticle free film at good deposition rates. The application of pulsed high voltage to the substrate placed at the exit of the duct is treated theoretically, and a model is proposed which gives a good agreement with the experimental concentration profile for a silicon surface coated and simultaneously implanted with titanium.

Carbon nitride materials have been the focus of research efforts worldwide. Most materials studied have been amorphous, with only a few groups claiming to have found a crystalline material. We investigate the structure of amorphous carbon nitride solids produced by two different techniques: cathodic arc deposition and high dose nitrogen implantation of glassy carbon. Transmission electron microscopy, electron energy loss spectroscopy, energy filtered electron diffraction, Rutherford backscattering, infrared, and Raman spectroscopy are all used to derive structural information. It was found that the carbon nitride materials prepared by both techniques have similar structures and [N]/[C] ratios (0.3). The materials appear to have a primarily sp2 bonded carbon structure with a smaller bond length than found in amorphous carbon. This is explained by nitrogen substituting into ‘‘rings’’ to a saturation level of about one nitrogen per three carbon atoms. We have found no evidence for a C3N4 crystalline structur...

Molecular dynamics simulations of amorphous diamond thin film growth were carried out using a Stillinger-Weber potential parameterised to describe both sp2 and sp3 carbon bonding. The effects of the incident beam energy and angle and substrate temperature were investigated. It was found that in very thin films, not deep enough to allow the generation of compressive stress, atomic impacts lead to the formation of a graphitic surface layer. Results also indicate that the optimal conditions for the formation of tetrahedral sp3 structures involve low incident beam energy and low substrate temperatures.

Tetrahedral amorphous carbon (ta-C) is a dense form of amorphous carbon with a structure consisting of a highly tetrahedral bonding network. Approximately 20% of the atoms in ta-C are ${\mathit{sp}}^{2}$ hybridized and the presence of these sites plays an important role in the electrical and optical properties of the material. In the present investigation, we use 50 keV ${\mathrm{C}}^{+}$ and 200 keV ${\mathrm{Xe}}^{+}$ ion implantation to damage the structure in a controlled manner. The structure of the ta-C following ion irradiation is monitored using the dose dependence of the electrical conductivity, Raman spectroscopy, electron diffraction, and electron energy-loss spectroscopy. It is shown that the damage is predominantly reflected in an increased concentration of ${\mathit{sp}}^{2}$-bonded sites. With increasing dose, the structure is observed to change from an essentially tetrahedral network containing ${\mathit{sp}}^{2}$ sites as ``defects'' to an essentially ${\mathit{sp}}^{2}$-bonded structure in which there is a high degree of in-plane disorder combined with a regular stacking of the planes.

A method of improving the adhesion of carbon thin films deposited using a cathodic vacuum arc by the use of implantation at energies up to 20 keV is described. A detailed analysis of carbon films deposited onto silicon in this way is carried out using complementary techniques of transmission electron microscopy and x‐ray photoelectron spectroscopy (XPS) is presented. This analysis shows that an amorphous mixing layer consisting of carbon and silicon is formed between the grown pure carbon film and the crystalline silicon substrate. In the mixing layer, it is shown that some chemical bonding occurs between carbon and silicon. Damage to the underlying crystalline silicon substrate is observed and believed to be caused by interstitial implanted carbon atoms which XPS shows are not bonded to the silicon. The effectiveness of this technique is confirmed by scratch testing and by analysis with scanning electron microscopy which shows failure of the silicon substrate occurs before delamination of the carbon film.

The thin-film transistor is one of a family of field-effect transistors. They all operate in the same way: a gate modulates the conductance of a channel and the current saturates when the drain end is depleted of carriers. The authors introduce a source-gated transistor that overcomes some of the fundamental limitations of the field-effect transistor. The gate controls the supply of carriers and the current saturates when the source is depleted of carriers. The result is a thin-film transistor that can operate at lower voltages with larger gains and lower power dissipation. It should also preserve its characteristics with smaller dimensions.

Compensated back-channel inverted staggered TFTs have been made in a-Si:H. Donor impurities were implanted to form a good source and drain ohmic contact followed by an acceptor implant to compensate the channel region. TFTs have been made with no degradation of channel mobility due to the implants and a leakage current between source and drain comparable with the best TFTs made using conventional etched back-channel technology.

Bulk unipolar diodes with a wide range of barrier heights have been made in hydrogenated amorphous silicon by ion implantation. The precise concentration of dopant atoms that can be obtained when implanting into an amorphous substrate leads to accurate barrier height control. Compared with the alternative unipolar device, the Schottky diode, these devices should provide uniform, high barrier diodes for photodetectors, and very low barrier diodes for low-power, unbiased, mixers and detectors.

Electron-microscope images of the life and death of a nanotube device provide a better understanding of how such devices can fail.

Honeybees can be trained to locate landmines with an accuracy of better than 97.5%. Tracking them with laser-based radar could provide a fast and effective means of clearing the world’s minefields.

The question of what material or structure could withstand the intense radiation in a fusion reactor is just one of the challenges to be faced on the road to fusion-based energy production. The answer could be a wall of liquid lithium.

Publisher Summary This chapter focuses on the evolution of a carbon (C) thin films and maps out the significant contributions to the field by numerous research laboratories. The impact of the microstructure and growth on the optical and electrical properties is also examined in the chapter. New results show how ion implantation allows a methodology to delocalize gap states within these films. Carbon is unique in its structure by being able to form one of the strongest materials known to man—diamond—or one that is soft—graphite—by virtue of the way in which each atom bonds to another. All of these variations are made possible by the three different bond hybridizations that are available to carbon. Diamond-like carbon (DLC) should generally be reserved for polycrystalline or nanocrystalline carbon films, whereas amorphous carbon films should generally fall into the categories of polymer such as amorphous carbon (PAC), graphite-like amorphous carbon (GAC), diamond-like amorphous carbon (DAC), tetrahedral amorphous carbon (TAC), and nano-composite amorphous carbon (NAC).

It has been shown that useful current gains can be obtained in hot-electron device structures containing very thin chromium disilicide layers of nanometer dimensions in hydrogenated amorphous silicon [1]. The a-Si:H/a-CrSi2/a-Si:H device structure made using PECVD and sputtering techniques naturally forms a hot-electron transistor device where the electrons are emitted across a high potential barrier on one side of the silicide and are collected over a low barrier on the other. Recent results [2] have shown that current gains can be in excess of 40 in structures having a-CrSi2 bases ∼1 nm thick.Here we outline the relatively simple technology used to make these devices and examine their performance as phototransistors in which the photo-current is amplified by hot-electron transistor action. The speed of response can be maximised by operating the phototransistor with high electric field across the collector since it is the transit time of the photo-induced carriers that determines the response time. We show that these devices provide a useful new active element for large area amorphous silicon electronics.

Much interest has been shown in the use of tetrahedral amorphous carbon (ta-C) deposited by filtered cathodic arc as an inexpensive, easily produced, wide band-gap semiconductor in the fabrication of electronic devices. There has, however, been limited success in producing devices with properties that might make its use in electronic applications commercially viable, which in part may be due to the high density of electronic trap states as reflected in ta-C's rather high ESR signal of ∼ 1020 spin/g. Recent results at the University of Sydney suggest, however, that a new range of possibilities exist in the utilisation of these traps as a means of producing nonvolatile digital information storage. Devices with write times of 100 μs, read times of 100 ns, and effective memory retention times approaching 1 year, have been fabricated.