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Helmersson, Ulf, ProfessorORCID iD iconorcid.org/0000-0002-1744-7322
Publications (10 of 150) Show all publications
Viloan, R. P., Lundin, D., Keraudy, J. & Helmersson, U. (2020). Tuning the stress in TiN films by regulating the doubly charged ion fraction in a reactive HiPIMS discharge. Journal of Applied Physics, 127(10), Article ID 103302.
Open this publication in new window or tab >>Tuning the stress in TiN films by regulating the doubly charged ion fraction in a reactive HiPIMS discharge
2020 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 127, no 10, article id 103302Article in journal (Refereed) Published
Abstract [en]

In the present study, we investigate the impact of pulse power (Ppulse) on the ion flux and the properties of TiN films using reactive high-power impulse magnetron sputtering. Ppulse was adjusted in the range of 5–25 kW, while keeping the total average power constant through regulating the pulsing frequency. It is found that the required N2 flow, to produce stoichiometric TiN, decreases as Ppulse is increased, which is due to a decrease in the deposition rate. The plasma conditions when stoichiometric TiN is formed were investigated in detail. In situ ion mass spectrometry measurements of the ion energy distribution functions reveal two distinct ion populations, ions originating from sputtered atoms (Ti+, Ti2+, and N+) and ions originating from the working gas (Ar+, Ar2+, and N2+). The average ion energies (Eave) of the sputtered ions show an increase with increasing Ppulse, while Eave for the gas ions remains almost unaffected. The relative flux intensity Ti2+/Ti+ showed an increasing trend, from 0.28 to 0.47, as Ppulse was increased from 5 to 25 kW. The ion flux changes affect the growth of the TiN film such that 111-textured films are grown for low Ppulse while higher Ppulse results in mixed orientations. In addition, the hardness of the deposited film increases with increasing Ppulse, while the compressive film stress increases significantly at a higher Ppulse. In this way, optimum deposition conditions were identified at Ppulse = 8.3 kW, where a relatively low compressive stress of 0.89 GPa and high hardness of 22.67 GPa were measured.

Keywords
Magnetron sputtering, Mass spectrometry, Thin films, Plasma dynamics, Compressive stress
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-164241 (URN)10.1063/1.5134003 (DOI)
Available from: 2020-03-11 Created: 2020-03-11 Last updated: 2020-03-12Bibliographically approved
Viloan, R. P., Gu, J., Boyd, R., Keraudy, J., Li, L. & Helmersson, U. (2019). Bipolar high power impulse magnetron sputtering for energetic ion bombardment during TiN thin film growth without the use of a substrate bias. Thin Solid Films, 688, Article ID 137350.
Open this publication in new window or tab >>Bipolar high power impulse magnetron sputtering for energetic ion bombardment during TiN thin film growth without the use of a substrate bias
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2019 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 688, article id 137350Article in journal (Refereed) Published
Abstract [en]

The effect of applying a positive voltage pulse (Urev = 10–150 V) directly after the negative high power impulse magnetron sputtering (HiPIMS) pulse (bipolar HiPIMS) is investigated for the reactive sputter deposition of TiN thin films. Energy-resolved mass spectroscopy analyses are performed to gain insight in the effect on the ion energy distribution function of the various ions. It is demonstrated that the energy of a large fraction of the ions can be tuned by a reverse target potential and gain energy corresponding to the applied Urev. Microscopy observations and x-ray reflectometry reveal densification of the films which results in an increase in the film hardness from 23.9 to 34 GPa as well as an increase in compressive film stress from 2.1 GPa to 4.7 GPa when comparing conventional HiPIMS with bipolar HiPIMS (Urev = 150 V).

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
High power impulse magnetron sputtering, Bipolar HiPIMS, Ion energy distribution function tuning, Titanium nitride
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-158296 (URN)10.1016/j.tsf.2019.05.069 (DOI)000485256500006 ()2-s2.0-85067028388 (Scopus ID)
Available from: 2019-06-28 Created: 2019-06-28 Last updated: 2019-11-14Bibliographically approved
Ekeroth, S., Ikeda, S., Boyd, R., Münger, P., Shimizu, T. & Helmersson, U. (2019). Impact of nanoparticle magnetization on the 3D formation of dual-phase Ni/NiO nanoparticle-based nanotrusses. Journal of nanoparticle research, 21(11), Article ID 21:228.
Open this publication in new window or tab >>Impact of nanoparticle magnetization on the 3D formation of dual-phase Ni/NiO nanoparticle-based nanotrusses
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2019 (English)In: Journal of nanoparticle research, ISSN 1388-0764, E-ISSN 1572-896X, Vol. 21, no 11, article id 21:228Article in journal (Refereed) Published
Abstract [en]

Magnetic nanoparticles with average size 30 nm were utilized to build three-dimensional framework structures—nanotrusses. In dual-phase Ni/NiO nanoparticles, there is a strong correlation between the amount of magnetic Ni and the final size and shape of the nanotruss. As it decreases, the length of the individual nanowires within the trusses also decreases, caused by a higher degree of branching of the wires. The position and orientation of the non-magnetic material within the truss structure was also investigated for the different phase compositions. For lower concentrations of NiO phase, the electrically conducting Ni-wire framework is maintained through the preferential bonding between the Ni crystals. For larger concentrations of NiO phase, the Ni-wire framework is interrupted by the NiO. The ability to use nanoparticles that are only partly oxidized in the growth of nanotruss structures is of great importance. It opens the possibility for using not only magnetic metals such as pure Ni, Fe, and Co, but also to use dual-phase nanoparticles that can strongly increase the efficiency of e.g. catalytic electrodes and fuel cells.

Place, publisher, year, edition, pages
Springer-Verlag New York, 2019
Keywords
Ni, NiO, Nanotruss, Nanoparticle, Magnetic assembly
National Category
Materials Chemistry
Identifiers
urn:nbn:se:liu:diva-161747 (URN)10.1007/s11051-019-4661-8 (DOI)000494039300001 ()
Note

Funding agencies

Available from: 2019-11-08 Created: 2019-11-08 Last updated: 2019-11-19Bibliographically approved
Ekeroth, S., Münger, P., Boyd, R., Ekspong, J., Wågberg, T., Edman, L., . . . Helmersson, U. (2018). Catalytic Nanotruss Structures Realized by Magnetic Self-Assembly in Pulsed Plasma. Nano letters (Print), 18(5), 3132-3137
Open this publication in new window or tab >>Catalytic Nanotruss Structures Realized by Magnetic Self-Assembly in Pulsed Plasma
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2018 (English)In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 18, no 5, p. 3132-3137Article in journal (Refereed) Published
Abstract [en]

Tunable nanostructures that feature a high surface area are firmly attached to a conducting substrate and can be fabricated efficiently over significant areas, which are of interest for a wide variety of applications in, for instance, energy storage and catalysis. We present a novel approach to fabricate Fe nanoparticles using a pulsed-plasma process and their subsequent guidance and self-organization into well-defined nanostructures on a substrate of choice by the use of an external magnetic field. A systematic analysis and study of the growth procedure demonstrate that nondesired nanoparticle agglomeration in the plasma phase is hindered by electrostatic repulsion, that a polydisperse nanoparticle distribution is a consequence of the magnetic collection, and that the formation of highly networked nanotruss structures is a direct result of the polydisperse nanoparticle distribution. The nanoparticles in the nanotruss are strongly connected, and their outer surfaces are covered with a 2 nm layer of iron oxide. A 10 mu m thick nanotruss structure was grown on a lightweight, flexible and conducting carbon-paper substrate, which enabled the efficient production of H-2 gas from water splitting at a low overpotential of 210 mV and at a current density of 10 mA/cm(2).

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2018
Keywords
Nanotrusses; nanowires; nanoparticles; iron; electrocatalysis; pulsed sputtering
National Category
Materials Chemistry
Identifiers
urn:nbn:se:liu:diva-148107 (URN)10.1021/acs.nanolett.8b00718 (DOI)000432093200055 ()29624405 (PubMedID)
Funder
Knut and Alice Wallenberg Foundation, KAW 14.0276
Available from: 2018-05-30 Created: 2018-05-30 Last updated: 2019-11-11
Rodner, M., Puglisi, D., Helmersson, U., Ivanov, I. G., Yakimova, R., Uvdal, K., . . . Eriksson, J. (2018). Iron oxide nanoparticle decorated graphene for ultra-sensitive detection of volatile organic compounds. Paper presented at EUROSENSORS 2018. Proceedings, 2(13), Article ID 985.
Open this publication in new window or tab >>Iron oxide nanoparticle decorated graphene for ultra-sensitive detection of volatile organic compounds
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2018 (English)In: Proceedings, ISSN 2504-3900, Vol. 2, no 13, article id 985Article in journal (Refereed) Published
Abstract [en]

It has been found that two-dimensional materials, such as graphene, can be used as remarkable gas detection platforms as even minimal chemical interactions can lead to distinct changes in electrical conductivity. In this work, epitaxially grown graphene was decorated with iron oxide nanoparticles for sensor performance tuning. This hybrid surface was used as a sensing layer to detect formaldehyde and benzene at concentrations of relevance in air quality monitoring (low parts per billion). Moreover, the time constants could be drastically reduced using a derivative sensor signal readout, allowing detection at the sampling rates desired for air quality monitoring applications.

Place, publisher, year, edition, pages
MDPI, 2018
Keywords
epitaxial graphene; metal oxide nanoparticle; gas sensor; volatile organic compounds; benzene; formaldehyde; derivative sensor signal
National Category
Physical Chemistry
Identifiers
urn:nbn:se:liu:diva-162242 (URN)10.3390/proceedings2130985 (DOI)
Conference
EUROSENSORS 2018
Available from: 2019-11-25 Created: 2019-11-25 Last updated: 2020-01-14Bibliographically approved
Magnfält, D., Fillon, A., Boyd, R., Helmersson, U., Sarakinos, K. & Abadias, G. (2016). Compressive intrinsic stress originates in the grain boundaries of dense refractory polycrystalline thin films. Journal of Applied Physics, 119(5), 055305
Open this publication in new window or tab >>Compressive intrinsic stress originates in the grain boundaries of dense refractory polycrystalline thin films
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2016 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 119, no 5, p. 055305-Article in journal (Refereed) Published
Abstract [en]

Intrinsic stresses in vapor deposited thin films have been a topic of considerable scientific and technological interest owing to their importance for functionality and performance of thin film devices. The origin of compressive stresses typically observed during deposition of polycrystalline metal films at conditions that result in high atomic mobility has been under debate in the literature in the course of the past decades. In this study, we contribute towards resolving this debate by investigating the grain size dependence of compressive stress magnitude in dense polycrystalline Mo films grown by magnetron sputtering. Although Mo is a refractory metal and hence exhibits an intrinsically low mobility, low energy ion bombardment is used during growth to enhance atomic mobility and densify the grain boundaries. Concurrently, the lateral grain size is controlled by using appropriate seed layers on which Mo films are grown epitaxially. The combination of in situ stress monitoring with ex situ microstructural characterization reveals a strong, seemingly linear, increase of the compressive stress magnitude on the inverse grain size and thus provides evidence that compressive stress is generated in the grain boundaries of the film. These results are consistent with models suggesting that compressive stresses in metallic films deposited at high homologous temperatures are generated by atom incorporation into and densification of grain boundaries. However, the underlying mechanisms for grain boundary densification might be different from those in the present study where atomic mobility is intrinsically low. (C) 2016 AIP Publishing LLC.

Place, publisher, year, edition, pages
AMER INST PHYSICS, 2016
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-125810 (URN)10.1063/1.4941271 (DOI)000369900600028 ()
Note

Funding Agencies|COST Action "Highly Ionized Pulsed Plasmas" [MP0804]; Swedish Research Council VR [621-2014-4882]; Linkoping University via the "LiU Research Fellows" program.

The previous status of this article was Manuscript and the working title was Atom insertion into grain boundaries generates compressive intrinsic stress in polycrystalline thin films.

Available from: 2016-03-08 Created: 2016-03-04 Last updated: 2017-11-30Bibliographically approved
Eriksson, J., Puglisi, D., Strandqvist, C., Gunnarsson, R., Ekeroth, S., Ivanov, I. G., . . . Lloyd Spetz, A. (2016). Modified Epitaxial Graphene on SiC for Extremely Sensitive andSelective Gas Sensors. Paper presented at ICSCRM 2015, The International Conference on Silicon Carbide and Related Materials, 4-9 October 2015, Giardini Naxos, Italy. Materials Science Forum, 858, 1145-1148
Open this publication in new window or tab >>Modified Epitaxial Graphene on SiC for Extremely Sensitive andSelective Gas Sensors
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2016 (English)In: Materials Science Forum, ISSN 0255-5476, E-ISSN 1662-9752, Vol. 858, p. 1145-1148Article in journal (Refereed) Published
Abstract [en]

Two-dimensional materials offer a unique platform for sensing where extremely high sensitivity is a priority, since even minimal chemical interaction causes noticeable changes inelectrical conductivity, which can be used for the sensor readout. However, the sensitivity has to becomplemented with selectivity, and, for many applications, improved response- and recovery times are needed. This has been addressed, for example, by combining graphene (for sensitivity) with metal/oxides (for selectivity) nanoparticles (NP). On the other hand, functionalization or modification of the graphene often results in poor reproducibility. In this study, we investigate thegas sensing performance of epitaxial graphene on SiC (EG/SiC) decorated with nanostructured metallic layers as well as metal-oxide nanoparticles deposited using scalable thin-film depositiontechniques, like hollow-cathode pulsed plasma sputtering. Under the right modification conditions the electronic properties of the surface remain those of graphene, while the surface chemistry can betuned to improve sensitivity, selectivity and speed of response to several gases relevant for airquality monitoring and control, such as nitrogen dioxide, benzene, and formaldehyde.

Place, publisher, year, edition, pages
Trans Tech Publications Inc., 2016
Keywords
Epitaxial graphene, graphene hybrid materials, gas sensor
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-128078 (URN)10.4028/www.scientific.net/MSF.858.1145 (DOI)
Conference
ICSCRM 2015, The International Conference on Silicon Carbide and Related Materials, 4-9 October 2015, Giardini Naxos, Italy
Available from: 2016-05-16 Created: 2016-05-16 Last updated: 2018-03-26
Shimizu, T., Villamayor, M. M., Lundin, D. & Helmersson, U. (2016). Process stabilization by peak current regulation in reactive high-power impulse magnetron sputtering of hafnium nitride. Journal of Physics D: Applied Physics, 49(6), 065202
Open this publication in new window or tab >>Process stabilization by peak current regulation in reactive high-power impulse magnetron sputtering of hafnium nitride
2016 (English)In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 49, no 6, p. 065202-Article in journal (Refereed) Published
Abstract [en]

A simple and cost effective approach to stabilize the sputtering process in the transition zone during reactive high-power impulse magnetron sputtering (HiPIMS) is proposed. The method is based on real-time monitoring and control of the discharge current waveforms. To stabilize the process conditions at a given set point, a feedback control system was implemented that automatically regulates the pulse frequency, and thereby the average sputtering power, to maintain a constant maximum discharge current. In the present study, the variation of the pulse current waveforms over a wide range of reactive gas flows and pulse frequencies during a reactive HiPIMS process of Hf-N in an Ar-N2 atmosphere illustrates that the discharge current waveform is a an excellent indicator of the process conditions. Activating the reactive HiPIMS peak current regulation, stable process conditions were maintained when varying the N-2 flow from 2.1 to 3.5 sccm by an automatic adjustment of the pulse frequency from 600 Hz to 1150 Hz and consequently an increase of the average power from 110 to 270 W. Hf-N films deposited using peak current regulation exhibited a stable stoichiometry, a nearly constant power-normalized deposition rate, and a polycrystalline cubic phase Hf-N with (1 1 1)-preferred orientation over the entire reactive gas flow range investigated. The physical reasons for the change in the current pulse waveform for different process conditions are discussed in some detail.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2016
Keywords
reactive sputtering; HiPIMS; hafnium nitride; process control
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:liu:diva-124621 (URN)10.1088/0022-3727/49/6/065202 (DOI)000368099600016 ()
Note

Funding Agencies|Swedish Research Council (VR) [VR 621-2014-4882]; Japan Society for the Promotion of Science (JSPS) [26820327]; AMADA foundation [AF-2013028]; Bases Conversion Development Authority (BCDA), Philippines

Available from: 2016-02-09 Created: 2016-02-08 Last updated: 2017-11-30
Vermang, B., Timo Watjen, J., Fjallstrom, V., Rostvall, F., Edoff, M., Gunnarsson, R., . . . Flandre, D. (2015). Highly reflective rear surface passivation design for ultra-thin Cu(In,Ga) Se-2 solar cells. Thin Solid Films, 582, 300-303
Open this publication in new window or tab >>Highly reflective rear surface passivation design for ultra-thin Cu(In,Ga) Se-2 solar cells
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2015 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 582, p. 300-303Article in journal (Refereed) Published
Abstract [en]

Al2O3 rear surface passivated ultra-thin Cu(In,Ga)Se-2 (CIGS) solar cells with Mo nano-particles (NPs) as local rear contacts are developed to demonstrate their potential to improve optical confinement in ultra-thin CIGS solar cells. The CIGS absorber layer is 380 nm thick and the Mo NPs are deposited uniformly by an up-scalable technique and have typical diameters of 150 to 200 nm. The Al2O3 layer passivates the CIGS rear surface between the Mo NPs, while the rear CIGS interface in contact with the Mo NP is passivated by [Ga]/([Ga] + [In]) (GGI) grading. It is shown that photon scattering due to the Mo NP contributes to an absolute increase in short circuit current density of 3.4 mA/cm(2); as compared to equivalent CIGS solar cells with a standard back contact.

Place, publisher, year, edition, pages
Elsevier, 2015
Keywords
Ultra-thin films; Copper Indium Gallium Selenide; Aluminum oxide; Surface passivation layer; Molybdenum; Nanoparticles; Local contacts; Solar cells
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-117642 (URN)10.1016/j.tsf.2014.10.050 (DOI)000352225900064 ()
Note

Funding Agencies|European Commission via FP7 Marie Curie IEF [300998]; Knut and Alice Wallenberg Foundation [2012.0083]; Swedish Research Council via the Linkoping Linnaeus Environment LiLi-NFM [2008-6572]; European and Wallonia Region FEDER [ECP12020011678F]

Available from: 2015-05-12 Created: 2015-05-06 Last updated: 2017-12-04
Gunnarsson, R., Helmersson, U. & Pilch, I. (2015). Synthesis of titanium-oxide nanoparticles with size and stoichiometry control. Journal of nanoparticle research, 17(9), 353
Open this publication in new window or tab >>Synthesis of titanium-oxide nanoparticles with size and stoichiometry control
2015 (English)In: Journal of nanoparticle research, ISSN 1388-0764, E-ISSN 1572-896X, Vol. 17, no 9, p. 353-Article in journal (Refereed) Published
Abstract [en]

Ti-O nanoparticles have been synthesized via hollow cathode sputtering in an Ar-O-2 atmosphere using high power pulsing. It is shown that the stoichiometry and the size of the nanoparticles can be varied independently, the former through controlling the O-2 gas flow and the latter by the independent biasing of two separate anodes in the growth zone. Nanoparticles with diameters in the range of 25-75 nm, and with different Ti-O compositions and crystalline phases, have been synthesized.

Place, publisher, year, edition, pages
Springer Verlag (Germany), 2015
Keywords
Titanium dioxide; TiO2; Reactive sputtering; Size control; Composition control; Gas flow sputtering; Aerosols
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-121300 (URN)10.1007/s11051-015-3158-3 (DOI)000360245300002 ()
Note

Funding Agencies|Knut and Alice Wallenberg foundation [KAW 2014.0276]; Swedish Research Council via the Linkoping Linneaus Environment LiLi-NFM [2008-6572]

Available from: 2015-09-16 Created: 2015-09-14 Last updated: 2017-12-21
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ORCID iD: ORCID iD iconorcid.org/0000-0002-1744-7322

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