Publications by authors named "Lars Hultman"

55 Publications

Boridene: Two-dimensional MoB with ordered metal vacancies obtained by chemical exfoliation.

Science 2021 08;373(6556):801-805

Materials Design, Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden.

Extensive research has been invested in two-dimensional (2D) materials, typically synthesized by exfoliation of van der Waals solids. One exception is MXenes, derived from the etching of constituent layers in transition metal carbides and nitrides. We report the experimental realization of boridene in the form of single-layer 2D molybdenum boride sheets with ordered metal vacancies, MoBT (where T is fluorine, oxygen, or hydroxide surface terminations), produced by selective etching of aluminum and yttrium or scandium atoms from 3D in-plane chemically ordered (MoY)AlB and (MoSc)AlB in aqueous hydrofluoric acid. The discovery of a 2D transition metal boride suggests a wealth of future 2D materials that can be obtained through the chemical exfoliation of laminated compounds.
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http://dx.doi.org/10.1126/science.abf6239DOI Listing
August 2021

Out-Of-Plane Ordered Laminate Borides and Their 2D Ti-Based Derivative from Chemical Exfoliation.

Adv Mater 2021 Sep 5;33(38):e2008361. Epub 2021 Aug 5.

Thin Film Physics Division, Department of Physics, Chemistry and Biology (IFM), Linköping University, Linköping, SE-581 83, Sweden.

Exploratory theoretical predictions in uncharted structural and compositional space are integral to materials discoveries. Inspired by M SiB (T2) phases, the finding of a family of laminated quaternary metal borides, M' M″SiB , with out-of-plane chemical order is reported here. 11 chemically ordered phases as well as 40 solid solutions, introducing four elements previously not observed in these borides are predicted. The predictions are experimentally verified for Ti MoSiB , establishing Ti as part of the T2 boride compositional space. Chemical exfoliation of Ti MoSiB and select removal of Si and MoB sub-layers is validated by derivation of a 2D material, TiO Cl , of high yield and in the form of delaminated sheets. These sheets have an experimentally determined direct band gap of ≈4.1 eV, and display characteristics suitable for supercapacitor applications. The results take the concept of chemical exfoliation beyond currently available 2D materials, and expands the envelope of 3D and 2D candidates, and their applications.
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http://dx.doi.org/10.1002/adma.202008361DOI Listing
September 2021

Electrochemical Lithium Storage Performance of Molten Salt Derived VSnC MAX Phase.

Nanomicro Lett 2021 Jul 22;13(1):158. Epub 2021 Jul 22.

Engineering Laboratory of Advanced Energy Materials, Ningbo Institute of Industrial Technology, Chinese Academy of Sciences, Ningbo, 315201, Zhejiang, People's Republic of China.

MAX phases are gaining attention as precursors of two-dimensional MXenes that are intensively pursued in applications for electrochemical energy storage. Here, we report the preparation of VSnC MAX phase by the molten salt method. VSnC is investigated as a lithium storage anode, showing a high gravimetric capacity of 490 mAh g and volumetric capacity of 570 mAh cm as well as superior rate performance of 95 mAh g (110 mAh cm) at 50 C, surpassing the ever-reported performance of MAX phase anodes. Supported by operando X-ray diffraction and density functional theory, a charge storage mechanism with dual redox reaction is proposed with a Sn-Li (de)alloying reaction that occurs at the edge sites of VSnC particles where Sn atoms are exposed to the electrolyte followed by a redox reaction that occurs at VC layers with Li. This study offers promise of using MAX phases with M-site and A-site elements that are redox active as high-rate lithium storage materials.
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http://dx.doi.org/10.1007/s40820-021-00684-6DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8298715PMC
July 2021

Halogenated TiC MXenes with Electrochemically Active Terminals for High-Performance Zinc Ion Batteries.

ACS Nano 2021 Jan 8;15(1):1077-1085. Epub 2021 Jan 8.

Engineering Laboratory of Advanced Energy Materials, Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang 315201, China.

The class of two-dimensional metal carbides and nitrides known as MXenes offer a distinct manner of property tailoring for a wide range of applications. The ability to tune the surface chemistry for expanding the property space of MXenes is thus an important topic, although experimental exploration of surface terminals remains a challenge. Here, we synthesized TiC MXene with unitary, binary, and ternary halogen terminals, .., -Cl, -Br, -I, -BrI, and -ClBrI, to investigate the effect of surface chemistry on the properties of MXenes. The electrochemical activity of Br and I elements results in the extraordinary electrochemical performance of the MXenes as cathodes for aqueous zinc ion batteries. The -Br- and -I-containing MXenes, ., TiCBr and TiCI, exhibit distinct discharge platforms with considerable capacities of 97.6 and 135 mAh·g. TiC(BrI) and TiC(ClBrI) exhibit dual discharge platforms with capacities of 117.2 and 106.7 mAh·g. In contrast, the previously discovered MXenes TiCCl and TiC(OF) exhibit no discharge platforms and only ∼50% of capacities and energy densities of TiCBr. These results emphasize the effectiveness of the Lewis-acidic-melt etching route for tuning the surface chemistry of MXenes and also show promise for expanding the MXene family toward various applications.
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http://dx.doi.org/10.1021/acsnano.0c07972DOI Listing
January 2021

A general Lewis acidic etching route for preparing MXenes with enhanced electrochemical performance in non-aqueous electrolyte.

Nat Mater 2020 Aug 13;19(8):894-899. Epub 2020 Apr 13.

Engineering Laboratory of Advanced Energy Materials, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, China.

Two-dimensional carbides and nitrides of transition metals, known as MXenes, are a fast-growing family of materials that have attracted attention as energy storage materials. MXenes are mainly prepared from Al-containing MAX phases (where A = Al) by Al dissolution in F-containing solution; most other MAX phases have not been explored. Here a redox-controlled A-site etching of MAX phases in Lewis acidic melts is proposed and validated by the synthesis of various MXenes from unconventional MAX-phase precursors with A elements Si, Zn and Ga. A negative electrode of TiC MXene material obtained through this molten salt synthesis method delivers a Li storage capacity of up to 738 C g (205 mAh g) with high charge-discharge rate and a pseudocapacitive-like electrochemical signature in 1 M LiPF carbonate-based electrolyte. MXenes prepared via this molten salt synthesis route may prove suitable for use as high-rate negative-electrode materials for electrochemical energy storage applications.
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http://dx.doi.org/10.1038/s41563-020-0657-0DOI Listing
August 2020

Atomic-Scale Tuning of Graphene/Cubic SiC Schottky Junction for Stable Low-Bias Photoelectrochemical Solar-to-Fuel Conversion.

ACS Nano 2020 Apr 7;14(4):4905-4915. Epub 2020 Apr 7.

Department of Physics, Chemistry and Biology (IFM), Linköping University, 58183 Linköping, Sweden.

Engineering tunable graphene-semiconductor interfaces while simultaneously preserving the superior properties of graphene is critical to graphene-based devices for electronic, optoelectronic, biomedical, and photoelectrochemical applications. Here, we demonstrate this challenge can be surmounted by constructing an interesting atomic Schottky junction epitaxial growth of high-quality and uniform graphene on cubic SiC (3C-SiC). By tailoring the graphene layers, the junction structure described herein exhibits an atomic-scale tunable Schottky junction with an inherent built-in electric field, making it a perfect prototype to systematically comprehend interfacial electronic properties and transport mechanisms. As a proof-of-concept study, the atomic-scale-tuned Schottky junction is demonstrated to promote both the separation and transport of charge carriers in a typical photoelectrochemical system for solar-to-fuel conversion under low bias. Simultaneously, the as-grown monolayer graphene with an extremely high conductivity protects the surface of 3C-SiC from photocorrosion and energetically delivers charge carriers to the loaded cocatalyst, achieving a synergetic enhancement of the catalytic stability and efficiency.
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http://dx.doi.org/10.1021/acsnano.0c00986DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7304924PMC
April 2020

Compromising Science by Ignorant Instrument Calibration-Need to Revisit Half a Century of Published XPS Data.

Angew Chem Int Ed Engl 2020 Mar 2;59(13):5002-5006. Epub 2020 Mar 2.

Thin Film Physics Division, Department of Physics, Chemistry, and Biology (IFM), Linköping University, 581 83, Linköping, Sweden.

X-ray photoelectron spectroscopy (XPS) is an indispensable technique in modern materials science for the determination of chemical bonding as evidenced by more than 10 000 XPS papers published annually. A literature survey reveals that in the vast majority of cases an incorrect referencing of the binding energy scale is used, neglecting warnings that have been formulated from the early days of the technique. Consequences for the data reliability are disastrous and decades of XPS work require revisiting. The purpose of this Viewpoint is to highlight the existing problems, review the criticism and suggest ways forward.
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http://dx.doi.org/10.1002/anie.201916000DOI Listing
March 2020

Multielemental single-atom-thick layers in nanolaminated V(Sn, ) C ( = Fe, Co, Ni, Mn) for tailoring magnetic properties.

Proc Natl Acad Sci U S A 2020 Jan 26;117(2):820-825. Epub 2019 Dec 26.

Engineering Laboratory of Advanced Energy Materials, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, 315201 Ningbo, Zhejiang, China;

Tailoring of individual single-atom-thick layers in nanolaminated materials offers atomic-level control over material properties. Nonetheless, multielement alloying in individual atomic layers in nanolaminates is largely unexplored. Here, we report 15 inherently nanolaminated V( Sn)C ( = Fe, Co, Ni, Mn, and combinations thereof, with x ∼ 1/3) MAX phases synthesized by an alloy-guided reaction. The simultaneous occupancy of the 4 magnetic elements and Sn in the individual single-atom-thick A layers constitutes high-entropy MAX phase in which multielemental alloying exclusively occurs in the 2-dimensional (2D) A layers. V( Sn)C exhibit distinct ferromagnetic behavior that can be compositionally tailored from the multielement A-layer alloying. Density functional theory and phase diagram calculations are performed to understand the structure stability of these MAX phases. This 2D multielemental alloying approach provides a structural design route to discover nanolaminated materials and expand their chemical and physical properties. In fact, the magnetic behavior of these multielemental MAX phases shows strong dependency on the combination of various elements.
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http://dx.doi.org/10.1073/pnas.1916256117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6969549PMC
January 2020

Synthesis of (VSc)AlC i-MAX phase and VC MXene scrolls.

Nanoscale 2019 Aug;11(31):14720-14726

Thin Film Physics Division, Department of Physics, Chemistry, and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden.

We report the synthesis and characterization of a new laminated i-MAX phase, (V2/3Sc1/3)2AlC, with in-plane chemical ordering between the M-elements. We also present evidence for the solid solution (V2-xScx)2AlC, where x ≤ 0.05. Chemical etching of the Al and Sc results in a two-dimensional (2D) MXene counterpart: V2-xC from the latter phase. Furthermore, etching with HF yields single-sheet MXene of flat morphology, while LiF + HCl gives MXene scrolls. We also show a 4× reduction in etching time for (V2-xScx)2AlC compared to V2AlC, suggesting that traces of Sc changes the phase stability, and make the material more susceptible to etching. The results show a path for improved control of MXene synthesis and morphology, which may be applicable also for other MAX/MXene systems.
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http://dx.doi.org/10.1039/c9nr02354bDOI Listing
August 2019

Single-Atom-Thick Active Layers Realized in Nanolaminated Ti(AlCu)C and Its Artificial Enzyme Behavior.

ACS Nano 2019 08 24;13(8):9198-9205. Epub 2019 Jul 24.

Engineering Laboratory of Advanced Energy Materials, Ningbo Institute of Materials Technology and Engineering , Chinese Academy of Sciences , Ningbo , Zhejiang 315201 , China.

A Ti(AlCu)C phase with Cu atoms with a degree of ordering in the A plane is synthesized through the A site replacement reaction in CuCl molten salt. The weakly bonded single-atom-thick Cu layers in a Ti(AlCu)C MAX phase provide actives sites for catalysis chemistry. As-synthesized Ti(AlCu)C presents unusual peroxidase-like catalytic activity similar to that of natural enzymes. A fabricated Ti(AlCu)C/chitosan/glassy carbon electrode biosensor prototype also exhibits a low detection limit in the electrochemical sensing of HO. These results have broad implications for property tailoring in a nanolaminated MAX phase by replacing the A site with late transition elements.
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http://dx.doi.org/10.1021/acsnano.9b03530DOI Listing
August 2019

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

J Am Chem Soc 2019 Mar 7;141(11):4730-4737. Epub 2019 Mar 7.

Engineering Laboratory of Advanced Energy Materials , Ningbo Institute of Industrial Technology, Chinese Academy of Sciences , Ningbo , Zhejiang 315201 , China.

Nanolaminated materials are important because of their exceptional properties and wide range of applications. Here, we demonstrate a general approach to synthesizing 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 the Zn element from molten ZnCl and the Al element in MAX phase precursors (TiAlC, TiAlC, TiAlN, and VAlC), novel MAX phases TiZnC, TiZnC, TiZnN, and VZnC were synthesized. When employing excess ZnCl, Cl-terminated MXenes (such as TiCCl and TiCCl) were derived by a subsequent exfoliation of TiZnC and TiZnC due to the strong Lewis acidity of molten ZnCl. 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 preparing MXenes through an HF-free chemical approach.
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http://dx.doi.org/10.1021/jacs.9b00574DOI Listing
March 2019

Efficient and Tunable Electroluminescence from In Situ Synthesized Perovskite Quantum Dots.

Small 2019 Feb 28;15(8):e1804947. Epub 2019 Jan 28.

Department of Physics, Chemistry and Biology (IFM), Linköping University, Linköping, 58183, Sweden.

Semiconductor quantum dots (QDs) are among the most promising next-generation optoelectronic materials. QDs are generally obtained through either epitaxial or colloidal growth and carry the promise for solution-processed high-performance optoelectronic devices such as light-emitting diodes (LEDs), solar cells, etc. Herein, a straightforward approach to synthesize perovskite QDs and demonstrate their applications in efficient LEDs is reported. The perovskite QDs with controllable crystal sizes and properties are in situ synthesized through one-step spin-coating from perovskite precursor solutions followed by thermal annealing. These perovskite QDs feature size-dependent quantum confinement effect (with readily tunable emissions) and radiative monomolecular recombination. Despite the substantial structural inhomogeneity, the in situ generated perovskite QDs films emit narrow-bandwidth emission and high color stability due to efficient energy transfer between nanostructures that sweeps away the unfavorable disorder effects. Based on these materials, efficient LEDs with external quantum efficiencies up to 11.0% are realized. This makes the technologically appealing in situ approach promising for further development of state-of-the-art LED systems and other optoelectronic devices.
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http://dx.doi.org/10.1002/smll.201804947DOI Listing
February 2019

A Tungsten-Based Nanolaminated Ternary Carbide: (W,Ti)C.

Inorg Chem 2019 Jan 4;58(2):1100-1106. Epub 2019 Jan 4.

Department of Materials Science and Engineering , Drexel University , Philadelphia , Pennsylvania 19104 , United States.

Nanolamellar transition metal carbides are gaining increasing interests because of the recent developments of their two-dimensional (2D) derivatives and promising performance for a variety of applications from energy storage, catalysis to transparent conductive coatings, and medicine. To develop more novel 2D materials, new nanolaminated structures are needed. Here we report on a tungsten-based nanolaminated ternary phase, (W,Ti)C, synthesized by an Al-catalyzed reaction of W, Ti, and C powders at 1600 °C for 4 h, under flowing argon. X-ray and neutron diffraction, along with Z-contrast scanning transmission electron microscopy, were used to determine the atomic structure, ordering, and occupancies. This phase has a layered hexagonal structure ( P6 /mmc) with lattice parameters, a = 3.00880(7) Å, and c = 19.5633(6) Å and a nominal chemistry of (W,Ti)C (actual chemistry, WTiC). The structure is comprised of layers of pure W that are also twin planes with two adjacent atomic layers of mixed W and Ti, on either side. The use of Al as a catalyst for synthesizing otherwise difficult to make phases, could in turn lead to the discovery of a large family of nonstoichiometric ternary transition metal carbides, synthesized at relatively low temperatures and shorter times.
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http://dx.doi.org/10.1021/acs.inorgchem.8b02226DOI Listing
January 2019

Origin of Chemically Ordered Atomic Laminates ( i-MAX): Expanding the Elemental Space by a Theoretical/Experimental Approach.

ACS Nano 2018 Aug 23;12(8):7761-7770. Epub 2018 Jul 23.

Thin Film Physics, Department of Physics, Chemistry and Biology (IFM) , Linköping University , SE-581 83 Linköping , Sweden.

With increased chemical diversity and structural complexity comes the opportunities for innovative materials possessing advantageous properties. Herein, we combine predictive first-principles calculations with experimental synthesis, to explore the origin of formation of the atomically laminated i-MAX phases. By probing (Mo M) AC (where M = Sc, Y and A = Al, Ga, In, Si, Ge, In), we predict seven stable i-MAX phases, five of which should have a retained stability at high temperatures. (MoSc)GaC and (MoY)GaC were experimentally verified, displaying the characteristic in-plane chemical order of Mo and Sc/Y and Kagomé-like ordering of the A-element. We suggest that the formation of i-MAX phases requires a significantly different size of the two metals, and a preferable smaller size of the A-element. Furthermore, the population of antibonding orbitals should be minimized, which for the metals herein (Mo and Sc/Y) means that A-elements from Group 13 (Al, Ga, In) are favored over Group 14 (Si, Ge, Sn). Using these guidelines, we foresee a widening of elemental space for the family of i-MAX phases and expect more phases to be synthesized, which will realize useful properties. Furthermore, based on i-MAX phases as parent materials for 2D MXenes, we also expect that the range of MXene compositions will be expanded.
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http://dx.doi.org/10.1021/acsnano.8b01774DOI Listing
August 2018

Self-Healing in Carbon Nitride Evidenced As Material Inflation and Superlubric Behavior.

ACS Appl Mater Interfaces 2018 May 3;10(19):16238-16243. Epub 2018 May 3.

Thin Film Physics Division, Department of Physics (IFM) , Linköping University , Linköping SE-581 83 , Sweden.

All known materials wear under extended mechanical contacting. Superlubricity may present solutions, but is an expressed mystery in C-based materials. We report negative wear of carbon nitride films; a wear-less condition with mechanically induced material inflation at the nanoscale and friction coefficient approaching ultralow values (0.06). Superlubricity in carbon nitride is expressed as C-N bond breaking for reduced coupling between graphitic-like sheets and eventual N desorption. The transforming surface layer acts as a solid lubricant, whereas the film bulk retains its high elasticity. The present findings offer new means for materials design at the atomic level, and for property optimization in wear-critical applications like magnetic reading devices or nanomachines.
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http://dx.doi.org/10.1021/acsami.8b03055DOI Listing
May 2018

Effects of N₂ Partial Pressure on Growth, Structure, and Optical Properties of GaN Nanorods Deposited by Liquid-Target Reactive Magnetron Sputter Epitaxy.

Nanomaterials (Basel) 2018 Apr 7;8(4). Epub 2018 Apr 7.

Thin Film Physics Division, Department of Physics, Chemistry, and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden.

GaN nanorods, essentially free from crystal defects and exhibiting very sharp band-edge luminescence, have been grown by reactive direct-current magnetron sputter epitaxy onto Si (111) substrates at a low working pressure of 5 mTorr. Upon diluting the reactive N₂ working gas with a small amount of Ar (0.5 mTorr), we observed an increase in the nanorod aspect ratio from 8 to ~35, a decrease in the average diameter from 74 to 35 nm, and a two-fold increase in nanorod density. With further dilution (Ar = 2.5 mTorr), the aspect ratio decreased to 14, while the diameter increased to 60 nm and the nanorod density increased to a maximum of 2.4 × 10⁸ cm. Yet, lower N₂ partial pressures eventually led to the growth of continuous GaN films. The observed morphological dependence on N₂ partial pressure is explained by a change from N-rich to Ga-rich growth conditions, combined with reduced GaN-poisoning of the Ga-target as the N₂ gas pressure is reduced. Nanorods grown at 2.5 mTorr N₂ partial pressure exhibited a high intensity 4 K photoluminescence neutral donor bound exciton transitions (D⁰X) peak at ~3.479 eV with a full-width-at-half-maximum of 1.7 meV. High-resolution transmission electron microscopy corroborated the excellent crystalline quality of the nanorods.
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http://dx.doi.org/10.3390/nano8040223DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5923553PMC
April 2018

W-Based Atomic Laminates and Their 2D Derivative W C MXene with Vacancy Ordering.

Adv Mater 2018 May 6;30(21):e1706409. Epub 2018 Apr 6.

Thin Film Physics Division, Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83, Linköping, Sweden.

Structural design on the atomic level can provide novel chemistries of hybrid MAX phases and their MXenes. Herein, density functional theory is used to predict phase stability of quaternary i-MAX phases with in-plane chemical order and a general chemistry (W M ) AC, where M = Sc, Y (W), and A = Al, Si, Ga, Ge, In, and Sn. Of over 18 compositions probed, only two-with a monoclinic C2/c structure-are predicted to be stable: (W Sc ) AlC and (W Y ) AlC and indeed found to exist. Selectively etching the Al and Sc/Y atoms from these 3D laminates results in W C-based MXene sheets with ordered metal divacancies. Using electrochemical experiments, this MXene is shown to be a new, promising catalyst for the hydrogen evolution reaction. The addition of yet one more element, W, to the stable of M elements known to form MAX phases, and the synthesis of a pure W-based MXene establishes that the etching of i-MAX phases is a fruitful path for creating new MXene chemistries that has hitherto been not possible, a fact that perforce increases the potential of tuning MXene properties for myriad applications.
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http://dx.doi.org/10.1002/adma.201706409DOI Listing
May 2018

Long Electron-Hole Diffusion Length in High-Quality Lead-Free Double Perovskite Films.

Adv Mater 2018 May 30;30(20):e1706246. Epub 2018 Mar 30.

Department of Physics, Chemistry, and Biology (IFM), Linköping University, Linköping, SE-581 83, Sweden.

Developing environmentally friendly perovskites has become important in solving the toxicity issue of lead-based perovskite solar cells. Here, the first double perovskite (Cs AgBiBr ) solar cells using the planar structure are demonstrated. The prepared Cs AgBiBr films are composed of high-crystal-quality grains with diameters equal to the film thickness, thus minimizing the grain boundary length and the carrier recombination. These high-quality double perovskite films show long electron-hole diffusion lengths greater than 100 nm, enabling the fabrication of planar structure double perovskite solar cells. The resulting solar cells based on planar TiO exhibit an average power conversion efficiency over 1%. This work represents an important step forward toward the realization of environmentally friendly solar cells and also has important implications for the applications of double perovskites in other optoelectronic devices.
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http://dx.doi.org/10.1002/adma.201706246DOI Listing
May 2018

Enhanced TiTaN diffusion barriers, grown by a hybrid sputtering technique with no substrate heating, between Si(001) wafers and Cu overlayers.

Sci Rep 2018 Mar 29;8(1):5360. Epub 2018 Mar 29.

Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Franz-Josef-Strasse 18, A-8700, Leoben, Austria.

We compare the performance of conventional DC magnetron sputter-deposited (DCMS) TiN diffusion barriers between Cu overlayers and Si(001) substrates with TiTaN barriers grown by hybrid DCMS/high-power impulse magnetron sputtering (HiPIMS) with substrate bias synchronized to the metal-rich portion of each pulse. DCMS power is applied to a Ti target, and HiPIMS applied to Ta. No external substrate heating is used in either the DCMS or hybrid DCMS/HiPIMS process in order to meet future industrial thermal-budget requirements. Barrier efficiency in inhibiting Cu diffusion into Si(001) while annealing for 1 hour at temperatures between 700 and 900 °C is investigated using scanning electron microscopy, X-ray diffraction, four-point-probe sheet resistance measurements, transmission electron microscopy, and energy-dispersive X-ray spectroscopy profiling. TiTaN barriers are shown to prevent large-scale Cu diffusion at temperatures up to 900 °C, while conventional TiN barriers fail at ≤700 °C. The improved performance of the TiTaN barrier is due to film densification resulting from HiPIMS pulsed irradiation of the growing film with synchronized Ta ions. The heavy ion bombardment dynamically enhances near-surface atomic mixing during barrier-layer deposition.
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http://dx.doi.org/10.1038/s41598-018-23782-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5876326PMC
March 2018

Influence of InAlN Nanospiral Structures on the Behavior of Reflected Light Polarization.

Nanomaterials (Basel) 2018 Mar 12;8(3). Epub 2018 Mar 12.

Thin Film Physics Division, Department of Physics, Chemistry, and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden.

The influence of structural configurations of indium aluminum nitride (InAlN) nanospirals, grown by reactive magnetron sputter epitaxy, on the transformation of light polarization are investigated in terms of varying structural chirality, growth temperatures, titanium nitride (TiN) seed (buffer) layer thickness, nanospiral thickness, and pitch. The handedness of reflected circularly polarized light in the ultraviolet-visible region corresponding to the chirality of nanospirals is demonstrated. A high degree of circular polarization (P) value of 0.75 is obtained from a sample consisting of 1.2 μm InAlN nanospirals grown at 650 °C. A film-like structure is formed at temperatures lower than 450 °C. At growth temperatures higher than 750 °C, less than 0.1 In-content is incorporated into the InAlN nanospirals. Both cases reveal very low P. A red shift of wavelength at P peak is found with increasing nanospiral pitch in the range of 200-300 nm. The P decreases to 0.37 for two-turn nanospirals with total length of 0.7 μm, attributed to insufficient constructive interference. A branch-like structure appears on the surface when the nanospirals are grown longer than 1.2 μm, which yields a low P around 0.5, caused by the excessive scattering of incident light.
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http://dx.doi.org/10.3390/nano8030157DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5869648PMC
March 2018

Phase formation of nanolaminated MoAuC and Mo(AuGa)C by a substitutional reaction within Au-capped MoGaC and MoGaC thin films.

Nanoscale 2017 Nov;9(45):17681-17687

Thin Film Physics Division, Department of Physics, Chemistry, and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden.

Au-containing nanolaminated carbides MoAuC and Mo(AuGa)C were synthesized by a thermally induced substitutional reaction in MoGaC and MoGaC, respectively. The Au substitution of the Ga layers in the structures was observed using cross-sectional high-resolution scanning transmission electron microscopy. Expansion of c lattice parameters was also observed in the Au-containing phases compared to the original phases. Energy dispersive spectroscopy detected residual Ga in Au-substituted layers of both phases with a peculiar Ga in-plane ordering for Au : Ga = 9 : 1 ratio along the Au-Ga layers in Mo(AuGa)C. These results indicate a generalization of the Au substitution reaction for the A elements in MAX phases.
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http://dx.doi.org/10.1039/c7nr03663aDOI Listing
November 2017

Selective-area growth of single-crystal wurtzite GaN nanorods on SiO/Si(001) substrates by reactive magnetron sputter epitaxy exhibiting single-mode lasing.

Sci Rep 2017 10 5;7(1):12701. Epub 2017 Oct 5.

Thin Film Physics Division, Department of Physics, Chemistry, and Biology (IFM), Linköping University, SE-581 83, Linköping, Sweden.

Selective-area growth (SAG) of single-crystal wurtzite GaN nanorods (NRs) directly onto Si(001) substrates with un-etched native SiO amorphous layer, assisted by a patterning TiN mask fabricated by nanosphere lithography (NSL), has been realized by reactive magnetron sputter epitaxy (MSE). The GaN NRs were grown vertically to the substrate surface with the growth direction along c-axis in the well-defined nano-opening areas. A 5-step structural and morphological evolution of the SAG NRs observed at different sputtering times depicts a comprehensive growth model, listed in sequence as: formation of a polycrystalline wetting layer, predominating c-axis oriented nucleation, coarsening and coalescence of multi-islands, single NR evolution, and finally quasi-equilibrium crystal shape formation. Room-temperature cathodoluminescence spectroscopy shows a strong GaN bandedge emission with a uniform luminescence across the NRs, indicating that the SAG NRs are grown with high quality and purity. In addition, single-longitudinal-mode lasing, attributed to well-faceted NR geometry forming a Fabry-Pérot cavity, was achieved by optical pumping, paving a way for fabricating high-performance laser optoelectronics using MSE.
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http://dx.doi.org/10.1038/s41598-017-12702-yDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5629253PMC
October 2017

Resolving mass spectral overlaps in atom probe tomography by isotopic substitutions - case of TiSiN.

Ultramicroscopy 2018 01 12;184(Pt A):51-60. Epub 2017 Aug 12.

Department of Physics, Chemistry and Biology (IFM), Linköping University, Linköping SE-581 83, Sweden.

Mass spectral overlaps in atom probe tomography (APT) analyses of complex compounds typically limit the identification of elements and microstructural analysis of a material. This study concerns the TiSiN system, chosen because of severe mass-to-charge-state ratio overlaps of the N and Si peaks as well as the N and Si peaks. By substituting N with N, mass spectrum peaks generated by ions composed of one or more N atoms will be shifted toward higher mass-to-charge-state ratios, thereby enabling the separation of N from the predominant Si isotope. We thus resolve thermodynamically driven Si segregation on the nanometer scale in cubic phase TiSiN thin films for Si contents 0.08 ≤ x ≤ 0.19 by APT, as corroborated by transmission electron microscopy. The APT analysis yields a composition determination that is in good agreement with energy dispersive X-ray spectroscopy and elastic recoil detection analyses. Additionally, a method for determining good voxel sizes for visualizing small-scale fluctuations is presented and demonstrated for the TiSiN system.
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http://dx.doi.org/10.1016/j.ultramic.2017.08.004DOI Listing
January 2018

Prediction and synthesis of a family of atomic laminate phases with Kagomé-like and in-plane chemical ordering.

Sci Adv 2017 07 19;3(7):e1700642. Epub 2017 Jul 19.

Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden.

The enigma of MAX phases and their hybrids prevails. We probe transition metal () alloying in MAX phases for metal size, electronegativity, and electron configuration, and discover ordering in these MAX hybrids, namely, (VZr)AlC and (MoY)AlC. Predictive theory and verifying materials synthesis, including a judicious choice of alloying M from groups III to VI and periods 4 and 5, indicate a potentially large family of thermodynamically stable phases, with Kagomé-like and in-plane chemical ordering, and with incorporation of elements previously not known for MAX phases, including the common Y. We propose the structure to be monoclinic 2/. As an extension of the work, we suggest a matching set of novel MXenes, from selective etching of the A-element. The demonstrated structural design on simultaneous two-dimensional (2D) and 3D atomic levels expands the property tuning potential of functional materials.
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http://dx.doi.org/10.1126/sciadv.1700642DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5517111PMC
July 2017

Synthesis of TiAuC, TiAuC and TiIrC by noble metal substitution reaction in TiSiC for high-temperature-stable Ohmic contacts to SiC.

Nat Mater 2017 08 1;16(8):814-818. Epub 2017 May 1.

Department of Physics, Chemistry, and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden.

The large class of layered ceramics encompasses both van der Waals (vdW) and non-vdW solids. While intercalation of noble metals in vdW solids is known, formation of compounds by incorporation of noble-metal layers in non-vdW layered solids is largely unexplored. Here, we show formation of TiAuC and TiAuC phases with up to 31% lattice swelling by a substitutional solid-state reaction of Au into TiSiC single-crystal thin films with simultaneous out-diffusion of Si. TiIrC is subsequently produced by a substitution reaction of Ir for Au in TiAuC. These phases form Ohmic electrical contacts to SiC and remain stable after 1,000 h of ageing at 600 °C in air. The present results, by combined analytical electron microscopy and ab initio calculations, open avenues for processing of noble-metal-containing layered ceramics that have not been synthesized from elemental sources, along with tunable properties such as stable electrical contacts for high-temperature power electronics or gas sensors.
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http://dx.doi.org/10.1038/nmat4896DOI Listing
August 2017

Two-dimensional MoC MXene with divacancy ordering prepared from parent 3D laminate with in-plane chemical ordering.

Nat Commun 2017 04 25;8:14949. Epub 2017 Apr 25.

Thin Film Physics, Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden.

The exploration of two-dimensional solids is an active area of materials discovery. Research in this area has given us structures spanning graphene to dichalcogenides, and more recently 2D transition metal carbides (MXenes). One of the challenges now is to master ordering within the atomic sheets. Herein, we present a top-down, high-yield, facile route for the controlled introduction of ordered divacancies in MXenes. By designing a parent 3D atomic laminate, (MoSc)AlC, with in-plane chemical ordering, and by selectively etching the Al and Sc atoms, we show evidence for 2D MoC sheets with ordered metal divacancies and high electrical conductivities. At ∼1,100 F cm, this 2D material exhibits a 65% higher volumetric capacitance than its counterpart, MoC, with no vacancies, and one of the highest volumetric capacitance values ever reported, to the best of our knowledge. This structural design on the atomic scale may alter and expand the concept of property-tailoring of 2D materials.
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http://dx.doi.org/10.1038/ncomms14949DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5413966PMC
April 2017

Dislocation-pipe diffusion in nitride superlattices observed in direct atomic resolution.

Sci Rep 2017 04 6;7:46092. Epub 2017 Apr 6.

Bradley Department of Electrical and Computer Engineering and Department of Materials Science and Engineering, Virginia Tech, Blacksburg, VA 24061, USA.

Device failure from diffusion short circuits in microelectronic components occurs via thermally induced migration of atoms along high-diffusivity paths: dislocations, grain boundaries, and free surfaces. Even well-annealed single-grain metallic films contain dislocation densities of about 10 m; hence dislocation-pipe diffusion (DPD) becomes a major contribution at working temperatures. While its theoretical concept was established already in the 1950s and its contribution is commonly measured using indirect tracer, spectroscopy, or electrical methods, no direct observation of DPD at the atomic level has been reported. We present atomically-resolved electron microscopy images of the onset and progression of diffusion along threading dislocations in sequentially annealed nitride metal/semiconductor superlattices, and show that this type of diffusion can be independent of concentration gradients in the system but governed by the reduction of strain fields in the lattice.
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http://dx.doi.org/10.1038/srep46092DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5382674PMC
April 2017

Synthesis and properties of CS F thin films deposited by reactive magnetron sputtering in an Ar/SF discharge.

J Phys Condens Matter 2017 May 20;29(19):195701. Epub 2017 Mar 20.

Thin Film Physics Division, Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-58183 Linköping, Sweden.

A theoretical and experimental study on the growth and properties of a ternary carbon-based material, CS F , synthesized from SF and C as primary precursors is reported. The synthetic growth concept was applied to model the possible species resulting from the fragmentation of SF molecules and the recombination of S-F fragments with atomic C. The possible species were further evaluated for their contribution to the film growth. Corresponding solid CS F thin films were deposited by reactive direct current magnetron sputtering from a C target in a mixed Ar/SF discharge with different SF partial pressures ([Formula: see text]). Properties of the films were determined by x-ray photoelectron spectroscopy, x-ray reflectivity, and nanoindentation. A reduced mass density in the CS F films is predicted due to incorporation of precursor species with a more pronounced steric effect, which also agrees with the low density values observed for the films. Increased [Formula: see text] leads to decreasing deposition rate and increasing density, as explained by enhanced fluorination and etching on the deposited surface by a larger concentration of F/F species during the growth, as supported by an increment of the F relative content in the films. Mechanical properties indicating superelasticity were obtained from the film with lowest F content, implying a fullerene-like structure in CS F compounds.
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http://dx.doi.org/10.1088/1361-648X/aa67d2DOI Listing
May 2017

C 1s Peak of Adventitious Carbon Aligns to the Vacuum Level: Dire Consequences for Material's Bonding Assignment by Photoelectron Spectroscopy.

Chemphyschem 2017 Jun 11;18(12):1507-1512. Epub 2017 Apr 11.

Thin Film Physics Division, Department of Physics (IFM), Linköping University, SE-581 83, Linköping, Sweden.

The C 1s signal from ubiquitous carbon contamination on samples forming during air exposure, so called adventitious carbon (AdC) layers, is the most common binding energy (BE) reference in X-ray photoelectron spectroscopy studies. We demonstrate here, by using a series of transition-metal nitride films with different AdC coverage, that the BE of the C 1s peak EBF varies by as much as 1.44 eV. This is a factor of 10 more than the typical resolvable difference between two chemical states of the same element, which makes BE referencing against the C 1s peak highly unreliable. Surprisingly, we find that C 1s shifts correlate to changes in sample work function ϕSA , such that the sum EBF+ϕSA is constant at 289.50±0.15 eV, irrespective of materials system and air exposure time, indicating vacuum level alignment. This discovery allows for significantly better accuracy of chemical state determination than offered by the conventional methods. Our findings are not specific to nitrides and likely apply to all systems in which charge transfer at the AdC/substrate interface is negligible.
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http://dx.doi.org/10.1002/cphc.201700126DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5484993PMC
June 2017
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