Imperial College London

ProfessorRogerWhatmore

Faculty of EngineeringDepartment of Materials

Principal Research Fellow
 
 
 
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r.whatmore

 
 
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Royal School of MinesSouth Kensington Campus

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Summary

 

Publications

Publication Type
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240 results found

Peters JJP, Sanchez AM, Walker D, Whatmore R, Beanland Ret al., Quantitative high-dynamic-range electron diffraction of polar nanodomains in Pb2ScTaO6, Advanced Materials, ISSN: 0935-9648

Highly B‐site ordered Pb2ScTaO6 crystals are studied as a function of temperature via dielectric spectroscopy and in situ high‐dynamic‐range electron diffraction. The degree of ordering is examined on the local and macroscopic scale and is determined to be 76%. Novel analysis of the electron diffraction patterns provides structural information with two types of antiferroelectric displacements determined to be present in the polar structure. It is then found that a low‐temperature transition occurs on cooling at ≈210 K that is not present on heating. This phenomenon is discussed in terms of the freezing of dynamic polar nanodomains where a high density of domain walls creates a metastable state.

Journal article

McQuaid R, Campbell M, Whatmore R, Kumar A, Gregg JMet al., 2017, Injection and controlled motion of conducting domain walls in improper ferroelectric Cu-Cl boracite, Nature Communications, Vol: 8, ISSN: 2041-1723

Ferroelectric domain walls constitute a completely new class of sheet-like functional material. Moreover, since domain walls are generally writable, erasable, and mobile, they could be useful in functionally agile devices: for example, creating and moving conducting walls could make or break electrical connections in new forms of reconfigurable nanocircuitry. However, significant challenges exist: site-specific injection and annihilation of planar walls, which show robust conductivity, has not been easy to achieve. Here, we report the observation, mechanical writing and controlled movement of charged conducting domain walls in the improper ferroelectric Cu3B7O13Cl. Walls are straight, tens of microns long, and exist as a consequence of elastic compatibility conditions between specific domain pairs. We show that site-specific injection of conducting walls of up to hundreds of microns in length can be achieved through locally applied point-stress and, once created, that they can be moved and repositioned using applied electric fields.

Journal article

Keeney L, Downing C, Schmidt M, Pemble ME, Nicolosi V, Whatmore Ret al., 2017, Direct atomic scale determination of magnetic ion partition in a room temperature multiferroic material, Scientific Reports, Vol: 7, ISSN: 2045-2322

The five-layer Aurivillius phase Bi6TixFeyMnzO18 system is a rare example of a single-phase room temperature multiferroic material. To optimise its properties and exploit it for future memory storage applications, it is necessary to understand the origin of the room temperature magnetisation. In this work we use high resolution scanning transmission electron microscopy, EDX and EELS to discover how closely-packed Ti/Mn/Fe cations of similar atomic number are arranged, both within the perfect structure and within defect regions. Direct evidence for partitioning of the magnetic cations (Mn and Fe) to the central three of the five perovskite (PK) layers is presented, which reveals a marked preference for Mn to partition to the central layer. We infer this is most probably due to elastic strain energy considerations. The observed increase (>8%) in magnetic cation content at the central PK layers engenders up to a 90% increase in potential ferromagnetic spin alignments in the central layer and this could be significant in terms of creating pathways to the long-range room temperature magnetic order observed in this distinct and intriguing material system.

Journal article

Faraz A, Maity T, Schmidt M, Deepak N, Roy S, Pemble ME, Whatmore RW, Keeney Let al., 2016, Direct visualization of magnetic-field-induced magnetoelectric switching in multiferroic aurivillius phase thin films, JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Vol: 100, Pages: 975-987, ISSN: 0002-7820

Multiferroic materials displaying coupled ferroelectric and ferromagnetic order parameters could provide a means for data storage whereby bits could be written electrically and read magnetically, or vice versa. Thin films of Aurivillius phase Bi6Ti2.8Fe1.52Mn0.68O18, previously prepared by a chemical solution deposition (CSD) technique, are multiferroics demonstrating magnetoelectric coupling at room temperature. Here, we demonstrate the growth of a similar composition, Bi6Ti2.99Fe1.46Mn0.55O18, via the liquid injection chemical vapor deposition technique. High-resolution magnetic measurements reveal a considerably higher in-plane ferromagnetic signature than CSD grown films (MS=24.25 emu/g (215 emu/cm3), MR=9.916 emu/g (81.5 emu/cm3), HC=170 Oe). A statistical analysis of the results from a thorough microstructural examination of the samples, allows us to conclude that the ferromagnetic signature can be attributed to the Aurivillius phase, with a confidence level of 99.95%. In addition, we report the direct piezoresponse force microscopy visualization of ferroelectric switching while going through a full in-plane magnetic field cycle, where increased volumes (8.6% to 14% compared with 4% to 7% for the CSD-grown films) of the film engage in magnetoelectric coupling and demonstrate both irreversible and reversible magnetoelectric domain switching.

Journal article

Keeney L, Schmidt M, Amann A, Maity T, Deepak N, Ahmad F, Petkov N, Roy S, Pemble ME, Whatmore RWet al., 2016, 25. Novel approaches for genuine single phase room temperaturemagnetoelectric multiferroics, Nanoscale Ferroelectrics and Multiferroics: Key Processing and Characterization Issues, and Nanoscale Effects, Volume I & II, Publisher: Wiley, ISBN: 978-1-118-93575-0

This chapter reviews approaches currently under investigation for the fabrication of single-phase magnetoelectric multiferroics, from bulk ceramics to those in thin-film form. It presents an approach of inserting magnetic ions into the Aurivillius phase, layer-structured ferroelectric materials, whereby thin films of average composition Bi6Ti2.8Fe1.52Mn0.68O18 (B6TFMO) demonstrate room temperature ferroelectricity, ferromagnetism, and magnetoelectric coupling. The chapter also discusses the importance of careful microstructural analysis of the materials and the application of a statistical model to determine a confidence level that the observed effects are from genuine single-phase magnetoelectric multiferroics. It reviews how careful phase analysis and statistical treatment of the data confirmed that the B6TFMO phase is a single-phase multiferroic to a confidence level of 99.5%. Finally, it summarizes how direct evidence of magnetoelectric coupling in the B6TFMO thin films was obtained. This review demonstrates that with materials development and design, the development of room temperature multiferroic materials can be achieved.

Book chapter

Skiadopoulou S, Kamba S, Drahokoupil J, Kroupa J, Deepak N, Pemble ME, Whatmore RWet al., 2015, Comment on "Interesting Evidence for Template-Induced Ferroelectric Behavior in Ultra-Thin Titanium Dioxide Films Grown on (110) Neodymium Gallium Oxide Substrates", ADVANCED FUNCTIONAL MATERIALS, Vol: 26, Pages: 642-646, ISSN: 1616-301X

X-ray diffraction, second-harmonic generation and infrared reflectance investigations reveal no evidence for a polar phase or ferroelectric phase transition in 1.6% tensile strained anatase TiO2 thin films. This indicates that the previously-reported potential ferroelectric behaviour, observed using piezoelectric force microscopy, may have been defect related, or the polar distortion is too small to detect using these methods.

Journal article

Douglas AM, Kumar A, Whatmore RW, Gregg JMet al., 2015, Local conductance: A means to extract polarization and depolarizing fields near domain walls in ferroelectrics, Applied Physics Letters, Vol: 107, ISSN: 0003-6951

Conducting atomic force microscopy images of bulk semiconducting BaTiO3 surfaces show clearstripe domain contrast. High local conductance correlates with strong out-of-plane polarization(mapped independently using piezoresponse force microscopy), and current-voltage characteristicsare consistent with dipole-induced alterations in Schottky barriers at the metallic tip-ferroelectricinterface. Indeed, analyzing current-voltage data in terms of established Schottky barrier modelsallows relative variations in the surface polarization, and hence the local domain structure, to bedetermined. Fitting also reveals the signature of surface-related depolarizing fields concentratednear domain walls. Domain information obtained from mapping local conductance appears to bemore surface-sensitive than that from piezoresponse force microscopy. In the right materials systems,local current mapping could therefore represent a useful complementary technique for evaluatingpolarization and local electric fields with nanoscale resolution

Journal article

Deepak N, Carolan P, Keeney L, Zhang PF, Pemble ME, Whatmore RWet al., 2015, Bismuth Self-Limiting Growth of Ultrathin BiFeO3 Films, CHEMISTRY OF MATERIALS, Vol: 27, Pages: 6508-6515, ISSN: 0897-4756

Bismuth ferrite (BiFeO3) is a widely studied material, because of its interesting multiferroic properties. Bismuth self-limiting growth of single-phase BiFeO3 (BFO) has previously been achieved using molecular beam epitaxy (MBE), but the growth of BFO by chemical vapor deposition (CVD) has proved to be very challenging, because of the volatile nature of bismuth. The growth window regarding temperature, pressure, and precursor flow rates that will give a pure perovskite BFO phase is normally very small. In this work, we have studied the metal–organic CVD (MOCVD) growth of epitaxial BFO thin films on SrTiO3 substrates and found that by carefully controlling the amount of the iron precursor, Fe(thd)3 (where thd = 2,2,6,6 tetra-methyl-3,5-heptanedionate), we were able to achieve bismuth self-liming growth, for the first time. The effect of the volume of the bismuth and iron precursors injected on the growth of BFO thin films is reported, and it has been found that the phase-pure films can be prepared when the Bi/Fe ratios are between 1.33 and 1.81 under temperature and pressure conditions of 650 °C and 10 mbar, respectively, and where the O2 gas flow was kept constant to 1000 sccm out of a total gas flow of 3000 sccm. Piezoresponse force microscopy (PFM) studies demonstrate the presence of bipolar switching in ultrathin BFO films.

Journal article

Deepak N, Carolan P, Keeney L, Pemble ME, Whatmore RWet al., 2015, Tunable nanoscale structural disorder in Aurivillius phase, n=3 Bi4Ti3O12 thin films and their role in the transformation to n=4, Bi5Ti3FeO15 phase, JOURNAL OF MATERIALS CHEMISTRY C, Vol: 3, Pages: 5727-5732, ISSN: 2050-7526

Naturally super-latticed Aurivillius phase ferroelectrics can accommodate various magnetic ions, opening up the possibility of making new room temperature multiferroics. Here, we studied the growth of single-phase Aurivillius phase Bi5Ti3FeO15 (BTFO) thin films, grown onto single crystalline SrTiO3 (STO) substrates, by doping Bi4Ti3O12 (BTO) with iron by liquid injection metal–organic chemical vapour deposition. The crystalline properties of the resulting films were characterized by X-ray diffraction and transmission electron microscopy. It has been found that the structural properties of the films depend strongly on the relative iron and titanium precursor injection volumes. Nanoscale structural disorder starts to occur in BTO films on the onset of iron precursor flow. A small iron precursor flow causes the formation of half-unit cells of BTFO inside BTO lattice, which in turns causes disorder in BTO films. This disorder can be tuned by varying iron content in the film. Atomic force microscopy shows how the growth mode switches from island growth to layer-by-layer growth mode as the composition changes from BTO to BTFO.

Journal article

Alpay SP, Mantese J, Trolier-McKinstry S, Zhang Q, Whatmore RWet al., 2014, Next-generation electrocaloric and pyroelectric materials for solid-state electrothermal energy interconversion, MRS Bulletin, Vol: 39, Pages: 1099-1111, ISSN: 0883-7694

Thin-film electrocaloric and pyroelectric sources for electrothermal energy interconversion have recently emerged as viable means for primary and auxiliary solid-state cooling and power generation. Two significant advances have facilitated this development: (1) the formation of high-quality polymeric and ceramic thin films with figures of merit that project system-level performance as a large percentage of Carnot efficiency and (2) the ability of these newer materials to support larger electric fields, thereby permitting operation at higher voltages. This makes the power electronic architectures more favorable for thermal to electric energy interconversion. Current research targets to adequately address commercial device needs including reduction of parasitic losses, increases in mechanical robustness, and the ability to form nearly freestanding elements with thicknesses in the range of 1–10 μm. This article describes the current state-of-the-art materials, thermodynamic cycles, and device losses and points toward potential lines of research that would lead to substantially better figures of merit for electrothermal energy interconversion.

Journal article

Dul'kin E, Salje EKH, Aktas O, Whatmore RW, Roth Met al., 2014, Ferroelectric precursor behavior of highly cation-ordered PbSc0.5Ta0.5O3 detected by acoustic emission: Tweed and polar nanoregions, Applied Physics Letters, Vol: 105, ISSN: 1077-3118

Highly ordered ferroelectric PbSc0.5Ta0.5O3 crystals were studied by acoustic emission over a widetemperature range. Acoustic emission was found at three characteristic temperatures: 330, 409, and600 K, which are close to those, known from the same disordered crystals, containing polarnanoregions. The microstructure in our crystals contains structural “tweed” rather thannanoregions. The coincidence of acoustic emission temperatures points towards a close structuralrelationship between nanoregions and “tweed.” Under electric field, these temperatures shiftsimilar to “critical end point” behavior. The obtained data prove that acoustic emission detectssignals in a wider parameter space than previously expected.

Journal article

Schmidt M, Amann A, Keeney L, Pemble ME, Holmes JD, Petkov N, Whatmore RWet al., 2014, Absence of Evidence not equal Evidence of Absence: Statistical Analysis of Inclusions in Multiferroic Thin Films, SCIENTIFIC REPORTS, Vol: 4, ISSN: 2045-2322

Journal article

Whatmore R, 2014, Characterisation of Pyroelectric Materials, Characterisation of Ferroelectric Bulk Materials and Thin Films, Editors: Cain, Publisher: Springer, Pages: 65-86, ISBN: 9781402093104

This book presents a comprehensive review of the most important methods used in the characterisation of piezoelectric, ferroelectric and pyroelectric materials.

Book chapter

Deepak N, Caro MA, Keeney L, Pemble ME, Whatmore RWet al., 2014, Interesting evidence for template-induced ferroelectric behavior in ultra-thin titanium dioxide films grown on (110) neodymium gallium oxide substrates, Advanced Functional Materials, Vol: 24, Pages: 2844-2851, ISSN: 1616-301X

The first evidence for room-temperature ferroelectric behavior in anatase-phase titanium dioxide (a-TiO2) is reported. Behavior strongly indicative of ferroelectric characteristics is induced in ultra-thin (20 nm to 80 nm) biaxially-strained epitaxial films of a-TiO2 deposited by liquid injection chemical vapor deposition onto (110) neodymium gallium oxide (NGO) substrates. The films exhibit significant orthorhombic strain, as analyzed by X-ray diffraction and high-resolution transmission electron microscopy. The films on NGO show a switchable dielectric spontaneous polarization when probed by piezoresponse force microscopy (PFM), the ability to retain polarization information written into the film using the PFM tip for extended periods (several hours) and at elevated temperatures (up to 100 °C) without significant loss, and the disappearance of the polarization at a temperature between 180 and 200 °C, indicative of a Curie temperature within this range. This combination of effects constitutes strong experimental evidence for ferroelectric behavior, which has not hitherto been reported in a-TiO2 and opens up the possibility for a range of new applications. A model is presented for the effects of large in-plane strains on the crystal structure of anatase which provides a possible explanation for the experimental observations.

Journal article

Whatmore R, Watton R, 2013, Pyroelectric Materials and Devices, Infrared Detectors and Emitters: Materials and Devices, Editors: Capper, Elliott, Publisher: Springer Science & Business Media, Pages: 99-148, ISBN: 9781461516071

Book chapter

Kesim MT, Zhang J, Trolier-McKinstry S, Mantese JV, Whatmore RW, Alpay SPet al., 2013, Pyroelectric response of lead zirconate titanate thin films on silicon: Effect of thermal stresses, JOURNAL OF APPLIED PHYSICS, Vol: 114, ISSN: 0021-8979

Ferroelectric lead zirconate titanate [Pb(ZrxTi1-xO)3, (PZT x:1-x)] has received considerable interest for applications related to uncooled infrared devices due to its large pyroelectric figures of merit near room temperature, and the fact that such devices are inherently ac coupled, allowing for simplified image post processing. For ferroelectric films made by industry-standard deposition techniques, stresses develop in the PZT layer upon cooling from the processing/growth temperature due to thermal mismatch between the film and the substrate. In this study, we use a non-linear thermodynamic model to investigate the pyroelectric properties of polycrystalline PZT thin films for five different compositions (PZT 40:60, PZT 30:70, PZT 20:80, PZT 10:90, PZT 0:100) on silicon as a function of processing temperature (25–800 °C). It is shown that the in-plane thermal stresses in PZT thin films alter the out-of-plane polarization and the ferroelectric phase transformation temperature, with profound effect on the pyroelectric properties. PZT 30:70 is found to have the largest pyroelectric coefficient (0.042 μC cm−2 °C−1, comparable to bulk values) at a growth temperature of 550 °C; typical to what is currently used for many deposition processes. Our results indicate that it is possible to optimize the pyroelectric response of PZT thin films by adjusting the Ti composition and the processing temperature, thereby, enabling the tailoring of material properties for optimization relative to a specific deposition process.

Journal article

Aktas O, Salje EKH, Crossley S, Lampronti GI, Whatmore RW, Mathur ND, Carpenter MAet al., 2013, Ferroelectric precursor behavior in PbSc0.5Ta0.5O3 detected by field-induced resonant piezoelectric spectroscopy, PHYSICAL REVIEW B, Vol: 88, ISSN: 2469-9950

Journal article

Keeney L, Maity T, Schmidt M, Amann A, Deepak N, Petkov N, Roy S, Pemble ME, Whatmore RWet al., 2013, Magnetic Field-Induced Ferroelectric Switching in Multiferroic Aurivillius Phase Thin Films at Room Temperature, Journal of the American Ceramic Society, Vol: 96, Pages: 2339-2357, ISSN: 0002-7820

Single-phase multiferroic materials are of considerable interest for future memory and sensing applications. Thin films of Aurivillius phase Bi7Ti3Fe3O21 and Bi6Ti2.8Fe1.52Mn0.68O18 (possessing six and five perovskite units per half-cell, respectively) have been prepared by chemical solution deposition on c-plane sapphire. Superconducting quantum interference device magnetometry reveal Bi7Ti3Fe3O21 to be antiferromagnetic (TN = 190 K) and weakly ferromagnetic below 35 K, however, Bi6Ti2.8Fe1.52Mn0.68O18 gives a distinct room-temperature in-plane ferromagnetic signature (Ms = 0.74 emu/g, μ0Hc =7 mT). Microstructural analysis, coupled with the use of a statistical analysis of the data, allows us to conclude that ferromagnetism does not originate from second phase inclusions, with a confidence level of 99.5%. Piezoresponse force microscopy (PFM) demonstrates room-temperature ferroelectricity in both films, whereas PFM observations on Bi6Ti2.8Fe1.52Mn0.68O18 show Aurivillius grains undergo ferroelectric domain polarization switching induced by an applied magnetic field. Here, we show for the first time that Bi6Ti2.8Fe1.52Mn0.68O18 thin films are both ferroelectric and ferromagnetic and, demonstrate magnetic field-induced switching of ferroelectric polarization in individual Aurivillius phase grains at room temperature.

Journal article

Deepak N, Zhang PF, Keeney L, Pemble ME, Whatmore RWet al., 2013, Atomic vapor deposition of bismuth titanate thin films, 21st IEEE ISAF, held Jointly with 11th ECAPD and 4th Conference on PFM and Nanoscale Phenomena in Polar Materials, Publisher: AMER INST PHYSICS, ISSN: 0021-8979

Conference paper

Varghese J, Ghoshal T, Deepak N, O'Regan C, Whatmore RW, Morris MA, Holmes JDet al., 2013, Fabrication of Arrays of Lead Zirconate Titanate (PZT) Nanodots via Block Copolymer Self-Assembly, CHEMISTRY OF MATERIALS, Vol: 25, Pages: 1458-1463, ISSN: 0897-4756

Journal article

Varghese J, Whatmore RW, Holmes JD, 2013, Ferroelectric nanoparticles, wires and tubes: synthesis, characterisation and applications, JOURNAL OF MATERIALS CHEMISTRY C, Vol: 1, Pages: 2618-2638, ISSN: 2050-7526

Journal article

Palizdar M, Comyn TP, Ward MB, Brown AP, Harrington JP, Kulkarni S, Keeney L, Roy S, Pemble M, Whatmore R, Quinn C, Kilcoyne SH, Bell AJet al., 2012, Crystallographic and magnetic identification of secondary phase in orientated Bi5Fe0.5Co0.5Ti3O15 ceramics, JOURNAL OF APPLIED PHYSICS, Vol: 112, ISSN: 0021-8979

Journal article

Zhang PF, Deepak N, Keeney L, Pemble ME, Whatmore RWet al., 2012, The structural and piezoresponse properties of c-axis-oriented Aurivillius phase Bi5Ti3FeO15 thin films deposited by atomic vapor deposition, APPLIED PHYSICS LETTERS, Vol: 101, ISSN: 0003-6951

Journal article

Keeney L, Kulkarni S, Deepak N, Schmidt M, Petkov N, Zhang PF, Cavill S, Roy S, Pemble ME, Whatmore RWet al., 2012, Room temperature ferroelectric and magnetic investigations and detailed phase analysis of Aurivillius phase Bi5Ti3Fe0.7Co0.3O15 thin films, JOURNAL OF APPLIED PHYSICS, Vol: 112, ISSN: 0021-8979

Journal article

Varghese J, O'Regan C, Deepak N, Whatmore RW, Homes JDet al., 2012, Surface Roughness Assisted Growth of Vertically Oriented Ferroelectric SbSI Nanorods, CHEMISTRY OF MATERIALS, Vol: 24, Pages: 3279-3284, ISSN: 0897-4756

Journal article

Keeney L, Groh C, Kulkarni S, Roy S, Pemble ME, Whatmore RWet al., 2012, Room temperature electromechanical and magnetic investigations of ferroelectric Aurivillius phase Bi5Ti3(FexMn1-x)O-15 (x=1 and 0.7) chemical solution deposited thin films, JOURNAL OF APPLIED PHYSICS, Vol: 112, ISSN: 0021-8979

Journal article

Varghese J, Barth S, Keeney L, Whatmore RW, Holmes JDet al., 2012, Nanoscale Ferroelectric and Piezoelectric Properties of Sb2S3 Nanowire Arrays, NANO LETTERS, Vol: 12, Pages: 868-872, ISSN: 1530-6984

Journal article

Deepak N, Zhang P, Keeney L, Pemble ME, Whatmore RWet al., 2012, Atomic Vapor Deposition of Bismuth Titanate thin films, 21st IEEE ISAF, held Jointly with 11th ECAPD and 4th Conference on PFM and Nanoscale Phenomena in Polar Materials, Publisher: IEEE, ISSN: 1099-4734

Conference paper

Zhang PF, Nagle RE, Deepak N, Povey IM, Gomeniuk YY, O'Connor E, Petkov N, Schmidt M, O'Regan TP, Cherkaoui K, Pemble ME, Hurley PK, Whatmore RWet al., 2011, The structural and electrical properties of the SrTa2O6/In0.53Ga0.47As/InP system, 17th International Conference on Insulating Films on Semiconductors, Publisher: ELSEVIER SCIENCE BV, Pages: 1054-1057, ISSN: 0167-9317

Conference paper

Correia TM, Kar-Narayan S, Young JS, Scott JF, Mathur ND, Whatmore RW, Zhang Qet al., 2011, PST thin films for electrocaloric coolers, JOURNAL OF PHYSICS D-APPLIED PHYSICS, Vol: 44, ISSN: 0022-3727

Journal article

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