arXiv subjects

Lars Hultman

Publications and source records attributed to Lars Hultman.

At least 37 records · Page 2Linked to original sources

arXiv2019

Element Replacement Approach by Reaction with Lewis Acidic Molten Salts to Synthesize Nanolaminated MAX Phases and MXenes

Nanolaminated materials are important because of their exceptional properties and wide range of applications. Here, we demonstrate a general approach to synthesize a series of Zn-based MAX phases and Cl-terminated MXenes originating from the replacement reaction between the MAX phase and the late transition metal halides. The approach is a top-down route that enables the late transitional element atom (Zn in the present case) to occupy the A site in the pre-existing MAX phase structure. Using this replacement reaction between Zn element from molten ZnCl2 and Al element in MAX phase precursors (Ti3AlC2, Ti2AlC, Ti2AlN, and V2AlC), novel MAX phases Ti3ZnC2, Ti2ZnC, Ti2ZnN, and V2ZnC were synthesized. When employing excess ZnCl2, Cl terminated MXenes (such as Ti3C2Cl2 and Ti2CCl2) were derived by a subsequent exfoliation of Ti3ZnC2 and Ti2ZnC due to the strong Lewis acidity of molten ZnCl2. These results indicate that A-site element replacement in traditional MAX phases by late transition metal halides opens the door to explore MAX phases that are not thermodynamically stable at high temperature and would be difficult to synthesize through the commonly employed powder metallurgy approach. In addition, this is the first time that exclusively Cl-terminated MXenes were obtained, and the etching effect of Lewis acid in molten salts provides a green and viable route to prepare MXenes through an HF-free chemical approach.

cond-mat.mtrl-sci↗

arXiv2019

Compositional dependence of epitaxial Tin+1SiCn MAX-phase thin films grown from a Ti3SiC2 compound target

We investigate sputtering of a Ti3SiC2 compound target at temperatures ranging from RT (no applied external heating) to 970 oC as well as the influence of the sputtering power at 850 oC for the deposition of Ti3SiC2 films on Al2O3(0001) substrates. Elemental composition obtained from time-of-flight energy elastic recoil detection analysis shows an excess of carbon in all films, which is explained by differences in angular distribution between C, Si and Ti, where C scatters the least during sputtering. The oxygen content is 2.6 at.% in the film deposited at RT and decreases with increasing deposition temperature, showing that higher temperatures favor high purity films. Chemical bonding analysis by X-ray photoelectron spectroscopy shows C-Ti and Si-C bonding in the Ti3SiC2 films and Si-Si bonding in the Ti3SiC2 compound target. X-ray diffraction reveals that the phases Ti3SiC2, Ti4SiC3, and Ti7Si2C5 can be deposited from a Ti3SiC2 compound target at substrate temperatures above 850 oC and with growth of TiC and the Nowotny phase Ti5Si3Cx at lower temperatures. High-resolution scanning transmission electron microscopy shows epitaxial growth of Ti3SiC2, Ti4SiC3, and Ti7Si2C5 on TiC at 970 oC. Four-point probe resistivity measurements give values in the range 120 to 450 micro-Ohm-cm and with the lowest values obtained for films containing Ti3SiC2, Ti4SiC3, and Ti7Si2C5.

cond-mat.mtrl-sci↗

arXiv2018

Chemical bonding in carbide MXene nanosheets

The chemical bonding in the carbide core and the surface chemistry in a new group of transition-metal carbides Tin+1Cn-Tx (n=1,2) called MXenes have been investigated by surface-sensitive valence band X-ray photoelectron spectroscopy. Changes in band structures of stacked nano sheets of different thicknesses are analyzed in connection to known hybridization regions of TiC and TiO2 that affect elastic and transport properties. By employing high excitation energy, the photoelectron cross-section for the C 2s - Ti 3d hybridization region at the bottom of the valence band is enhanced. As shown in this work, the O 2p and F 2p bands are shown to strongly depend both on the bond lengths to the surface groups and the adsorption sites. The effect of surface oxidation and Ar+ sputtering on the electronic structure is also discussed.

cond-mat.mtrl-sci↗

arXiv2018

Chemical Bonding in Epitaxial ZrB2 Studied by X-ray Spectroscopy

The chemical bonding in an epitaxial ZrB2 film is investigated by Zr K-edge (1s) X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies and compared to the ZrB2 compound target from which the film was synthesized as well as a bulk α-Zr reference. Quantitative analysis of X-ray Photoelectron Spectroscopy spectra reveals at the surface: ~5% O in the epitaxial ZrB2 film, ~19% O in the ZrB2 compound target and ~22% O in the bulk α-Zr reference after completed sputter cleaning. For the ZrB2 compound target, X-ray diffraction (XRD) shows weak but visible -111, 111, and 220 peaks from monoclinic ZrO2 together with peaks from ZrB2 and where the intensity distribution for the ZrB2 peaks show a randomly oriented target material. For the bulk α-Zr reference no peaks from any crystalline oxide were visible in the diffractogram recorded from the 0001-oriented metal. The Zr K-edge absorption from the two ZrB2 samples demonstrate more pronounced oscillations for the epitaxial ZrB2 film than in the bulk ZrB2 attributed to the high atomic ordering within the columns of the film. The XANES exhibits no pre-peak due to lack of p-d hybridization in ZrB2, but with a chemical shift towards higher energy of 4 eV in the film and 6 eV for the bulk compared to α-Zr (17.993 keV) from the charge-transfer from Zr to B. The 2 eV larger shift in bulk ZrB2 material suggests higher oxygen content than in the epitaxial film, which is supported by XPS. In EXAFS, the modelled cell-edge in ZrB2 is slightly smaller in the thin film (a=3.165 Å, c=3.520 Å) in comparison to the bulk target material (a=3.175 Å, c=3.540 Å) while in hexagonal closest-packed metal (α-phase, a=3.254 Å, c=5.147 Å).

cond-mat.mtrl-sci↗

arXiv2017

Bonding Structures of ZrHx Thin Films by X-ray Spectroscopy

The variation in local atomic structure and chemical bonding of ZrHx (x=0.15, 0.30, 1.16) magnetron sputtered thin films are investigated by Zr K-edge (1s) X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopies. A chemical shift of the Zr K-edge towards higher energy with increasing hydrogen content is observed due to charge-transfer and an ionic or polar covalent bonding component between the Zr 4d and the H 1s states with increasing valency for Zr. We find an increase in the Zr-Zr bond distance with increasing hydrogen content from 3.160 Å in the hexagonal closest-packed metal (alpha-phase) to 3.395 Å in the understoichiometric delta-ZrHx film (CaF2-type structure) with x=1.16 that largely resembles that of bulk delta-ZrH2. For yet lower hydrogen contents, the structures are mixed alpha and delta-phases, while sufficient hydrogen loading (x>1) yields a pure δ-phase that is understoichiometric, but thermodynamically stable. The change in the hydrogen content and strain is discussed in relation to the corresponding change of bond lengths, hybridizations, and trends in electrical resistivity.

cond-mat.mtrl-sci↗

arXiv2017

Electronic Properties and Bonding in ZrHx Thin Films Investigated by Valence-Band X-ray Photoelectron Spectroscopy

The electronic structure and chemical bonding in reactively magnetron sputtered ZrHx (x=0.15, 0.30, 1.16) thin films with oxygen content as low as 0.2 at% are investigated by 4d valence band, shallow 4p core-level and 3d core-level X-ray photoelectron spectroscopy. With increasing hydrogen content, we observe significant reduction of the 4d valence states close to the Fermi level as a result of redistribution of intensity towards the H 1s - Zr 4d hybridization region at about 6 eV below the Fermi level. For low hydrogen content (x=0.15, 0.30), the films consist of a superposition of hexagonal closest packed metal (alpha-phase)and understoichiometric delta-ZrHx (CaF2-type structure) phases, while for x=1.16, the film form single phase ZrHx that largely resembles that of stoichiometric delta-ZrH2 phase. We show that the cubic delta-ZrHx phase is metastable as thin film up to x=1.16 while for higher H-contents, the structure is predicted to be tetragonally distorted. For the investigated ZrH1.16 film, we find chemical shifts of 0.68 and 0.51 eV towards higher binding energies for the Zr 4p3/2 and 3d5/2 peak positions, respectively. Compared to the Zr metal binding energies of 27.26 and 178.87 eV, this signifies a charge-transfer from Zr to H atoms. The change in the electronic structure, spectral line shapes, and chemical shifts as function of hydrogen content is discussed in relation to the charge-transfer from Zr to H that affects the conductivity by charge redistribution in the valence band.

cond-mat.mtrl-sci↗

arXiv2016

First Measurements with New High-Resolution Gadolinium-GEM Neutron Detectors

European Spallation Source instruments like the macromolecular diffractometer, NMX, require an excellent neutron detection efficiency, high-rate capabilities, time resolution, and an unprecedented spatial resolution in the order of a few hundred micrometers over a wide angular range of the incoming neutrons. For these instruments solid converters in combination with Micro Pattern Gaseous Detectors (MPGDs) are a promising option. A GEM detector with gadolinium converter was tested on a cold neutron beam at the IFE research reactor in Norway. The μTPC analysis, proven to improve the spatial resolution in the case of $^{10}$B converters, is extended to gadolinium based detectors. For the first time, a Gd-GEM was successfully operated to detect neutrons with a measured efficiency of 11.8% at a wavelength of 2 Å and a position resolution better than 250 μm.

physics.ins-det↗

arXiv2015

The uTPC Method: Improving the Position Resolution of Neutron Detectors Based on MPGDs

Due to the Helium-3 crisis, alternatives to the standard neutron detection techniques are becoming urgent. In addition, the instruments of the European Spallation Source (ESS) require advances in the state of the art of neutron detection. The instruments need detectors with excellent neutron detection efficiency, high-rate capabilities and unprecedented spatial resolution. The Macromolecular Crystallography instrument (NMX) requires a position resolution in the order of 200 um over a wide angular range of incoming neutrons. Solid converters in combination with Micro Pattern Gaseous Detectors (MPGDs) are proposed to meet the new requirements. Charged particles rising from the neutron capture have usually ranges larger than several millimetres in gas. This is apparently in contrast with the requirements for the position resolution. In this paper, we present an analysis technique, new in the field of neutron detection, based on the Time Projection Chamber (TPC) concept. Using a standard Single-GEM with the cathode coated with 10B4C, we extract the neutron interaction point with a resolution of better than sigma = 200 um.

physics.ins-det↗

arXiv2015

Structure and bonding in amorphous iron carbide thin films

We investigate the amorphous structure, chemical bonding, and electrical properties of magnetron sputtered Fe1-xCx (0.21<x<0.72) thin films. X-ray, electron diffraction and transmission electron microscopy show that the Fe1-xCx films are amorphous nanocomposites, consisting of a two-phase domain structure with Fe-rich carbidic FeCy, and a carbon-rich matrix. Pair distribution function analysis indicates a close-range order similar to those of crystalline Fe3C carbides in all films with additional graphene-like structures at high carbon content (71.8 at% C). From X-ray photoelectron spectroscopy measurements, we find that the amorphous carbidic phase has a composition of 15-25 at% carbon that slightly increases with total carbon content. X-ray absorption spectra exhibit increasing number of unoccupied 3d states and decreasing number of C 2p states as a function of carbon content. These changes signify a systematic redistribution in orbital occupation due to charge-transfer effects at the domain-size dependent carbide/matrix interfaces. Four-point probe resistivity of the Fe1-xCx films increases exponentially with carbon content from 200 mu-Ohm-cm (x=0.21) to 1200 mu-Ohm-cm (x=0.72), and is found to depend on the total carbon content rather than the composition of the carbide. Our findings open new possibilities for modifying the resistivity of amorphous thin film coatings based on transition metal carbides by control of amorphous domain structures.

cond-mat.mtrl-sci↗

arXiv2014

Crystallization characteristics and chemical bonding properties of nickel carbide thin film nanocomposites

The crystal structure and chemical bonding of magnetron-sputtering deposited nickel carbide Ni$_{1-x}$C$_{x}$ (0.05$\leq$x$\leq$0.62) thin films have been investigated by high-resolution X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and soft X-ray absorption spectroscopy. By using X-ray as well as electron diffraction, we found carbon-containing hcp-Ni (hcp-NiC$_{y}$ phase), instead of the expected rhombohedral-Ni$_{3}$C. At low carbon content (4.9 at\%) the thin film consists of hcp-NiC$_{y}$ nanocrystallites mixed with a smaller amount of fcc-NiC$_{x}$. The average grain size is about 10-20 nm. With the increase of carbon content to 16.3 at\%, the film contains single-phase hcp-NiC$_{y}$ nanocrystallites with expanded lattice parameters. With further increase of carbon content to 38 at\%, and 62 at\%, the films transform to X-ray amorphous materials with hcp-NiC$_{y}$ and fcc-NiC$_{x }$ nanodomain structures in an amorphous carbon-rich matrix. Raman spectra of carbon indicate dominant $sp^{2}$ hybridization, consistent with photoelectron spectra that show a decreasing amount of C-Ni phase with increasing carbon content. The Ni $3d$ - C $2p$ hybridization in the hexagonal structure gives rise to the salient double-peak structure in Ni $2p$ soft X-ray absorption spectra at 16.3 at\% that changes with carbon content. We also show that the resistivity is not only governed by the amount of carbon, but increases by more than a factor of two when the samples transform from crystalline to amorphous.

cond-mat.mtrl-sci↗

arXiv2012

Neutron Diffraction Measurements and First Principles Study of Thermal Motion of Atoms in Select M_{n+1}AX_n and Binary MX Transition Metal Carbide Phases

Herein, we compare the thermal vibrations of atoms in select ternary carbides with the formula Mn+1AXn ("MAX phases," M = Ti, Cr; A = Al, Si, Ge; X = C, N) as determined from first principles phonon calculations to those obtained from high-temperature neutron powder diffraction studies. The transition metal carbides TiC, TaC, and WC are also studied to test our methodology on simpler carbides. Good qualitative and quantitative agreement is found between predicted and experimental values for the binary carbides. For all the MAX phases studied - Ti3SiC2, Ti3GeC2, Ti2AlN, Cr2GeC and Ti4AlN3 - density functional theory calculations predict that the A element vibrates with the highest amplitude and does so anisotropically with a higher amplitude within the basal plane, which is in line with earlier results from high-temperature neutron diffraction studies. In some cases, there are quantitative differences in the absolute values between the theoretical and experimental atomic displacement parameters, such as reversal of anisotropy or a systematic offset of temperature-dependent atomic displacement parameters. The mode-dependent Grüneisen parameters are also computed to explore the anharmonicity in the system.

cond-mat.mtrl-sci↗

arXiv2012

Multi-Grid Boron-10 detector for large area applications in neutron scattering science

The present supply of 3He can no longer meet the detector demands of the upcoming ESS facility and continued detector upgrades at current neutron sources. Therefore viable alternative technologies are required to support the development of cutting-edge instrumentation for neutron scattering science. In this context, 10B-based detectors are being developed by collaboration between the ESS, ILL, and Linköping University. This paper reports on progress of this technology and the prospects applying it in modern neutron scattering experiments. The detector is made-up of multiple rectangular gas counter tubes coated with B4C, enriched in 10B. An anode wire reads out each tube, thereby giving position of conversion in one of the lateral co-ordinates as well as in depth of the detector. Position resolution in the remaining co-ordinate is obtained by segmenting the cathode tube itself. Boron carbide films have been produced at Linköping University and a detector built at ILL. The characterization study is presented in this paper, including measurement of efficiency, effects of the fill gas species and pressure, coating thickness variation on efficiency and sensitivity to gamma-rays.

physics.ins-det↗

arXiv2012

The electronic-structure origin of the anisotropic thermopower of nanolaminated Ti3SiC2 determined by polarized x-ray spectroscopy and Seebeck measurements

Nanolaminated materials exhibit characteristic magnetic, mechanical, and thermoelectric properties, with large contemporary scientific and technological interest. Here, we report on the anisotropic Seebeck coefficient in nanolaminated Ti3SiC2 single-crystal thin films and trace the origin to anisotropies in element-specific electronic states. In bulk polycrystalline form, Ti3SiC2 has a virtually zero Seebeck coefficient over a wide temperature range. In contrast, we find that the in-plane (basal ab) Seebeck coefficient of Ti3SiC2, measured on single-crystal films has a substantial and positive value of 4-6 muV/K. Employing a combination of polarized angle-dependent x-ray spectroscopy and density functional theory we directly show electronic structure anisotropy in inherently nanolaminated Ti3SiC2 single-crystal thin films as a model system. The density of Ti 3d and C 2p states at the Fermi level in the basal ab-plane is about 40 % higher than along the c-axis. The Seebeck coefficient is related to electron and hole-like bands close to the Fermi level but in contrast to ground state density functional theory modeling, the electronic structure is also influenced by phonons that need to be taken into account. Positive contribution to the Seebeck coefficient of the element-specific electronic occupations in the basal plane is compensated by 73 % enhanced Si 3d electronic states across the laminate plane that give rise to a negative Seebeck coefficient in that direction. Strong phonon vibration modes with three to four times higher frequency along the c-axis than along the basal ab-plane also influence the electronic population and the measured spectra by the asymmetric average displacements of the Si atoms. These results constitute experimental evidence explaining why the average Seebeck coefficient of Ti3SiC2 in polycrystals is negligible over a wide temperature range.

cond-mat.mtrl-sci↗

arXiv2012

Electronic Structure and Chemical Bonding of Amorphous Chromium Carbide Thin Films

The microstructure, electronic structure, and chemical bonding of chromium carbide thin films with different carbon contents have been investigated with high-resolution transmission electron microscopy, electron energy loss spectroscopy and soft x-ray absorption-emission spectroscopies. Most of the films can be described as amorphous nanocomposites with non-crystalline CrCx in an amorphous carbon matrix. At high carbon contents, graphene-like structures are formed in the amorphous carbon matrix. At 47 at% carbon content, randomly oriented nanocrystallites are formed creating a complex microstructure of three components. The soft x-ray absorption-emission study shows additional peak structures exhibiting non-octahedral coordination and bonding.

cond-mat.mes-hall↗

arXiv2011

Anisotropy in the electronic structure of V2GeC investigated by soft x-ray emission spectroscopy and first-principles theory

The anisotropy of the electronic structure of ternary nanolaminate V2GeC is investigated by bulk-sensitive soft x-ray emission spectroscopy. The measured polarization-dependent emission spectra of V L2,3, C K, Ge M1 and Ge M2,3 in V2GeC are compared to those from monocarbide VC and pure Ge. The experimental emission spectra are interpreted with calculated spectra using ab initio density-functional theory including dipole transition matrix elements. Different types of covalent chemical bond regions are revealed; V 3d - C 2p bonding at -3.8 eV, Ge 4p - C 2p bonding at -6 eV and Ge 4p - C 2s interaction mediated via the V 3d orbitals at -11 eV below the Fermi level. We find that the anisotropic effects are high for the 4p valence states and the shallow 3d core levels of Ge, while relatively small anisotropy is detected for the V 3d states. The macroscopic properties of the V2GeC nanolaminate result from the chemical bonds with the anisotropic pattern as shown in this work.

cond-mat.mtrl-sci↗

arXiv2011

The electronic structure of GaN and Ga investigated by soft x-ray spectroscopy and first-principles methods

The electronic structure and chemical bonding of wurtzite-GaN investigated by N 1s soft x-ray absorption spectroscopy and N K, Ga M1, and Ga M2,3 emission spectroscopy is compared to that of pure Ga. The measurements are interpreted by calculated spectra using first-principles density-functional theory (DFT) including dipole transition matrix elements and additional on-site Coulomb interaction (WC-GGA+U). The Ga 4p - N 2p and Ga 4s - N 2p hybridization and chemical bond regions are identified at the top of the valence band between -1.0 and -2.0 and further down between -5.5 and -6.5 eV, respectively. In addition, N 2s - N 2p - Ga 4s and N 2s - N 2p - Ga 3d hybridization regions occur at the bottom of the valence band between -13 and -15 eV, and between -17.0 and -18.0 eV, respectively. A band-like satellite feature is also found around -10 eV in the Ga M1 and Ga M2,3 emission from GaN, but is absent in pure Ga and the calculated ground state spectra. The difference between the identified spectroscopic features of GaN and Ga are discussed in relation to the various hybridization regions calculated within band-structure methods.

cond-mat.mtrl-sci↗

arXiv2011

Electronic structure investigation of the cubic inverse perovskite Sc3AlN

The electronic structure and chemical bonding of the recently discovered inverse perovskite Sc3AlN, in comparison to ScN and Sc metal have been investigated by bulk-sensitive soft x-ray emission spectroscopy. The measured Sc L, N K, Al L1, and Al L2,3 emission spectra are compared with calculated spectra using first principle density-functional theory including dipole transition matrix elements. The main Sc 3d - N 2p and Sc 3d - Al 3p chemical bond regions are identified at -4 eV and -1.4 eV below the Fermi level, respectively. A strongly modified spectral shape of 3s states in the Al L2,3 emission from Sc3AlN in comparison to pure Al metal is found, which reflects the Sc 3d - Al 3p hybridization observed in the Al L1 emission. The differences between the electronic structure of Sc3AlN, ScN, and Sc metal are discussed in relation to the change of the conductivity and elastic properties.

cond-mat.mtrl-sci↗

arXiv2011

Electronic structure and chemical bonding of nc-TiC/a-C nanocomposites

The electronic structure of nanocrystalline (nc-) TiC/amorphous C nanocomposites has been investigated by soft x-ray absorption and emission spectroscopy. The measured spectra at the Ti 2p and C 1s thresholds of the nanocomposites are compared to those of Ti metal and amorphous C. The corresponding intensities of the electronic states for the valence and conduction bands in the nanocomposites are shown to strongly depend on the TiC carbide grain size. An increased charge-transfer between the Ti 3d-eg states and the C 2p states has been identified as the grain size decreases, causing an increased ionicity of the TiC nanocrystallites. It is suggested that the charge-transfer occurs at the interface between the nanocrystalline TiC and the amorphous C matrix and represents an interface bonding which may be essential for the understanding of the properties of nc-TiC/amorphous C and similar nanocomposites.

cond-mat.mes-hall↗