Himadri Majumdar | VTT Techn. Res. centre of Finland (original) (raw)
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Papers by Himadri Majumdar
Pulsed Laser Deposition (PLD) technique is applied to produce organic semiconductor (OS) rubrene ... more Pulsed Laser Deposition (PLD) technique is applied to produce organic semiconductor (OS) rubrene thin film for
spintronics applications. The use of organic material for spintronics is motivated by the advantages such as long spin
diffusion length due to low spin-orbit and hyperfine coupling, chemical tuning of electronic functionality, easy
structural modifications, ability of self-assembly and mechanical flexibility etc. However, a major drawback of OS is its low mobility compared to inorganic semiconductors. The PLD growth of rubrene aims on fabricating OS films under
more controlled environment to achieve higher crystalline order to improve its mobility and spin coherence length.
Among organic materials, rubrene reveals the highest hole mobility - up to 40 cm2/(V∙s) and can be exploited in organic
light-emitting diodes (OLEDs) or field-effect transistors (OFETs) 4. In this work the rubrene thin films are produced
from hardened pellets in vacuum using Nd:YAG pulsed laser operated at 1064 nm, 2 Hz and energy fluence around 0.2
J/cm2. For the reference rubrene samples on SiO2 glass the AFM data reveal continuous 5-7 nm thick films. The
amorphous structures are confirmed by XRD measurements and also Raman spectra which show signatures of both
tetracene and phenyl bands and a broadband at 1373 cm-1. The obtained results indicate that continuous, defect-free
rubrene films can be prepared by means of PLD for investigation of the spin polarization properties of organic-inorganic hybrids. Further studies are on the way to improve crystalline qualities of the rubrene films for less grain boundary
related defects and improved mobility and spin diffusion length.
The purpose of this work is to fabricate large-scale solution processable graphene-based films fr... more The purpose of this work is to fabricate large-scale solution processable graphene-based films from graphene oxide (GO) solution and to characterize the transport properties of these films. The graphene like film is produced by annealing of the GO film to form reduced graphene oxide (rGO) thin films. The conductive rGO thin films are useable as spacer layers in spin valves and as organic electrodes. Atomic Force Microscope (AFM) characterizations on the film thickness and morphology have been carried out and simple electrical transport studies performed on spin coated rGO thin films. We have fabricated rGO thin films ranging from few to tens of nanometers in thickness with conductivities in the order of 1-100 S/m. We also show that the morphology of the films play an important role in facilitating higher conductivities for rGO thin films.
Applied Physics Letters, 2006
The authors report on fabrication and characterization of a polymeric spin valve with the conjuga... more The authors report on fabrication and characterization of a polymeric spin valve with the conjugated polymer regioregular (poly 3-hexylthiophene) (RRP3HT) as the spacer layer. The device structure is La0.67Sr0.33MnO3 (LSMO)/polymer/Co, with half-metallic, spin-polarized LSMO acting as the spin-injecting electrode. The spin valve shows behavior similar to a magnetic tunnel junction though the nonmagnetic spacer layer (∼100 nm) is much thicker than the tunneling limit. They attribute this behavior to the formation of a thin spin-selective tunneling interface between LSMO and RRP3HT caused by RRP3HT, chemically attaching to LSMO as observed by x-ray photoelectron spectroscopy measurement. This gives rise to ∼80% magnetoresistance (MR) at 5 K and ∼1.5% MR at room temperature. They found that by introducing monolayer of different organic insulators between LSMO and RRP3HT the spin-selective interface is destroyed and the spin injection is reduced. Their results show that organic materials are promising candidates for spintronic applications.
Journal of Alloys and Compounds, 2006
We report spin polarized injection and transport in organic spin-valves made from both organic sm... more We report spin polarized injection and transport in organic spin-valves made from both organic small molecules and polymers. The devices with the structure La 0.67 Sr 0.33 MnO 3 (LSMO)/ organic spacer/ Co showed inverse magnetoresistance and low temperature operation with the vacuum evaporated πconjugated small molecule 8-hydroxyquinoline aluminium (Alq 3 ) while normal MR and high temperature operation was observed in devices with the πconjugated polymer poly (3-hexylthiophene). Due to the proximate values of the work functions of LSMO and Co with the highest occupied molecular orbital (HOMO) energy of the polymers, spin polarized carrier injection is much more efficient in the polymeric spin-valves compared to the organic spin-valves with Alq 3 where carrier injection is hindered due to greater barrier height. Efficient spin transport is also observed in polymeric spin-valves and can be attributed to longer conjugation in the polymeric chains compared to the small molecules.
Organic Electronics, 2005
We have observed bistability in single layer devices made from fullerenes (C60) mixed with polyst... more We have observed bistability in single layer devices made from fullerenes (C60) mixed with polystyrene (PS) and sandwiched between two Al electrodes. By merely changing the concentration of C60 in PS we found three distinctly different device properties, namely a true insulator, a bistable device switching between an OFF and an ON state having ON-OFF current ratios larger than 10 4 and a write-once, read-many times (WORM) device. An additional negative differential resistance (NDR) was observed in the ON state of both the bistable and WORM devices leading to multilevel switching capability of the devices. This opens up a wide range of application possibilities of such devices in disposable printable electronics.
Physical Review B, 2009
Magneto-electrical measurements were performed on diodes and bulk heterojunction solar cells (BHS... more Magneto-electrical measurements were performed on diodes and bulk heterojunction solar cells (BHSCs) to clarify the role of formation of coulombically bound electron-hole (e-h) pairs on the magnetoresistance (MR) response in organic thin film devices. BHSCs are suitable model systems because they effectively quench excitons but the probability of forming e-h pairs in them can be tuned over orders of magnitude by the choice of material and solvent in the blend. We have systematically varied the e-h recombination coefficients, which are directly proportional to the probability for the charge carriers to meet in space, and found that a reduced probability of electrons and holes meeting in space lead to disappearance of the MR. Our results clearly show that MR is a direct consequence of e-h pair formation. We also found that the MR line shape follows a power law-dependence of B 0.5 at higher fields.
We report on magnetotransport studies of regio regular poly (3-hexyl thiophene) (RRP3HT) based di... more We report on magnetotransport studies of regio regular poly (3-hexyl thiophene) (RRP3HT) based diodes and P3HT: 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]methanofullerene (PCBM) bulk heterojunction solar cells. While the P3HT diodes with non-magnetic electrodes ITO and Al show up to 16 % positive magnetoresistance (MR) under 300 mT magnetic field at room temperature, solar cells with much reduced electronhole pair (e-h) formation probability show almost negligible MR clearly suggesting that MR in organic semiconductors is strongly dependent on the probability of forming Coulombically bound e-h pairs. Voltage and temperature dependence of MR sign and magnitude is discussed for the diode devices which provides important insight of the scientifically puzzling yet technologically very promising magnetotransport properties of organic semiconductor based devices.
![Research paper thumbnail of Organic memory using [6,6]-phenyl-C61 butyric acid methyl ester: morphology, thickness and concentration dependence studies](https://attachments.academia-assets.com/50237876/thumbnails/1.jpg)
](Nanotechnology, 2008
Organic memory using [6,6]-phenyl-C 61 butyric acid methyl ester: morphology, thickness and conce... more Organic memory using [6,6]-phenyl-C 61 butyric acid methyl ester: morphology, thickness and concentration dependence studies. Nanotechnology, volume 19, number 3, 035203.
Applied Physics Letters, 2008
A r i L a i h o , H i m a d r i S . M a j u m d a r , J a y a n t a K . B a r a l , F r e d r i k... more A r i L a i h o , H i m a d r i S . M a j u m d a r , J a y a n t a K . B a r a l , F r e d r i k J a n s s o n , R o n a l d Österbacka, and Olli Ikkala. 2008. Tuning the electrical switching of polymer/fullerene nanocomposite thin film devices by control of morphology. Applied Physics Letters, volume 93, number 20, 203309.
Thin Solid Films, 2003
We have fabricated and characterized sandwich-type devices based on an oriented polythiophene der... more We have fabricated and characterized sandwich-type devices based on an oriented polythiophene derivative. Depending on the voltage sweep direction, the devices showed the presence of two conducting states. Switching between the two states and an associated memory have earlier been studied by current-voltage characteristics. In this article, we have studied impedance spectroscopy of the material to model the device. We have studied conductance switching by measuring capacitance of the device, and have probed the high-and low-conducting states of the device. It has been shown that voltage variation of dielectric constant of the material depends on the voltage sweep direction. We have also demonstrated that capacitance of the device can probe the two states for data-storage applications. ᮊ
We report observation of ferromagnetic (FM) ordering in organic semiconductors, namely regioregul... more We report observation of ferromagnetic (FM) ordering in organic semiconductors, namely regioregular poly (3-hexyl thiophene) (RRP3HT) and 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]methanofullerene (PCBM), in the temperature range of 5 -300 K in addition to magnetoresistance (OMAR) observed in the diodes made from the same materials. Particle induced x-ray emission spectroscopy confirms the presence of dilute magnetic impurities in the materials mainly as residues from the synthesis process. However, upon blending these two materials with FM signal, the FM ordering is suppressed by a huge paramagnetic (PM) signal indicating ground state charge transfer formation in the blend. Together with the magneto-transport studies, these results indicate that OMAR response is observed in a device only when the corresponding active materials are FM. In the diodes with P3HT:PCBM complex, that as a blend shows PM response, OMAR vanishes almost completely. We propose that ferromagnetism in the active material can have important correlation with the OMAR response in the diodes.
... Sayani Majumdar (Abo Akademi University and University of Turku). ... to the formation of a t... more ... Sayani Majumdar (Abo Akademi University and University of Turku). ... to the formation of a thin spin-selective tunneling interface between LSMO and RRP3HT caused by the chemical bonding between RRP3HT and LSMO as observed by x-ray photoelectron spectroscopy ...
We report giant magnetoresistance up to 150% at low bias current and low temperature as well as r... more We report giant magnetoresistance up to 150% at low bias current and low temperature as well as room temperature magnetoresistance in polymeric spin-valves having the structure La 0.67 Sr 0.33 MnO 3 (LSMO)/conjugated polymer/Co. The conjugated polymersregiorandom and regioregular poly(3-hexyl thiophene) were used as the spacer materials.
Conjugated polymers have been used in data-storage devices. A ''state'' has been written by apply... more Conjugated polymers have been used in data-storage devices. A ''state'' has been written by applying a voltage pulse. The state of the device has been ''read'' from the current under a small probe voltage ͑0.2 V͒. The polymer retained the state for more than 1 h which can be refreshed or erased at will. The stored space charges under a voltage pulse have been found to control the charge injection and hence the device current. Their slow relaxation process has resulted in the use of conjugated polymers in memory device applications. Hysteresis-type behavior has been observed in the current-voltage characteristics. The density of stored charges at the polymer layer near the metal/polymer interface has been found to depend on the voltage amplitude. The relaxation of the stored charges has been studied by applying two voltage pulses. By varying the delay between the two pulses, during which the space charges relax or redistribute, the time constant for charge relaxation has been calculated. The time constant was found to be independent of the density of the space charges or of the pace at which they were stored.
Journal of Materials Chemistry, 2006
In this paper, we present a new method that combines surface modification of indium-tin oxide (IT... more In this paper, we present a new method that combines surface modification of indium-tin oxide (ITO) and electropolymerization to prepare thin, sulfonated poly(aniline) (SPAN) films with good surface coverage. The surface modification enhances the growth of the SPAN film resulting in a better rectification signal than for SPAN films polymerized on unmodified ITO substrates. The films were characterized by FTIR, scanning electron microscopy (SEM) and atomic force microscopy (AFM). The sulfonation degree of SPAN was determined to 29% by X-ray photoelectron spectroscopy (XPS). UV-VIS spectroscopy shows that the pH sensitivity of SPAN is suppressed due to sulfonation of the polymer backbone. It is also shown that conversion of the SPAN film to the emeraldine salt (ES) form after polymerization is crucial for obtaining a high rectification signal. This is an important practical aspect in the preparation procedure of organic electronic devices.
Pulsed Laser Deposition (PLD) technique is applied to produce organic semiconductor (OS) rubrene ... more Pulsed Laser Deposition (PLD) technique is applied to produce organic semiconductor (OS) rubrene thin film for
spintronics applications. The use of organic material for spintronics is motivated by the advantages such as long spin
diffusion length due to low spin-orbit and hyperfine coupling, chemical tuning of electronic functionality, easy
structural modifications, ability of self-assembly and mechanical flexibility etc. However, a major drawback of OS is its low mobility compared to inorganic semiconductors. The PLD growth of rubrene aims on fabricating OS films under
more controlled environment to achieve higher crystalline order to improve its mobility and spin coherence length.
Among organic materials, rubrene reveals the highest hole mobility - up to 40 cm2/(V∙s) and can be exploited in organic
light-emitting diodes (OLEDs) or field-effect transistors (OFETs) 4. In this work the rubrene thin films are produced
from hardened pellets in vacuum using Nd:YAG pulsed laser operated at 1064 nm, 2 Hz and energy fluence around 0.2
J/cm2. For the reference rubrene samples on SiO2 glass the AFM data reveal continuous 5-7 nm thick films. The
amorphous structures are confirmed by XRD measurements and also Raman spectra which show signatures of both
tetracene and phenyl bands and a broadband at 1373 cm-1. The obtained results indicate that continuous, defect-free
rubrene films can be prepared by means of PLD for investigation of the spin polarization properties of organic-inorganic hybrids. Further studies are on the way to improve crystalline qualities of the rubrene films for less grain boundary
related defects and improved mobility and spin diffusion length.
The purpose of this work is to fabricate large-scale solution processable graphene-based films fr... more The purpose of this work is to fabricate large-scale solution processable graphene-based films from graphene oxide (GO) solution and to characterize the transport properties of these films. The graphene like film is produced by annealing of the GO film to form reduced graphene oxide (rGO) thin films. The conductive rGO thin films are useable as spacer layers in spin valves and as organic electrodes. Atomic Force Microscope (AFM) characterizations on the film thickness and morphology have been carried out and simple electrical transport studies performed on spin coated rGO thin films. We have fabricated rGO thin films ranging from few to tens of nanometers in thickness with conductivities in the order of 1-100 S/m. We also show that the morphology of the films play an important role in facilitating higher conductivities for rGO thin films.
Applied Physics Letters, 2006
The authors report on fabrication and characterization of a polymeric spin valve with the conjuga... more The authors report on fabrication and characterization of a polymeric spin valve with the conjugated polymer regioregular (poly 3-hexylthiophene) (RRP3HT) as the spacer layer. The device structure is La0.67Sr0.33MnO3 (LSMO)/polymer/Co, with half-metallic, spin-polarized LSMO acting as the spin-injecting electrode. The spin valve shows behavior similar to a magnetic tunnel junction though the nonmagnetic spacer layer (∼100 nm) is much thicker than the tunneling limit. They attribute this behavior to the formation of a thin spin-selective tunneling interface between LSMO and RRP3HT caused by RRP3HT, chemically attaching to LSMO as observed by x-ray photoelectron spectroscopy measurement. This gives rise to ∼80% magnetoresistance (MR) at 5 K and ∼1.5% MR at room temperature. They found that by introducing monolayer of different organic insulators between LSMO and RRP3HT the spin-selective interface is destroyed and the spin injection is reduced. Their results show that organic materials are promising candidates for spintronic applications.
Journal of Alloys and Compounds, 2006
We report spin polarized injection and transport in organic spin-valves made from both organic sm... more We report spin polarized injection and transport in organic spin-valves made from both organic small molecules and polymers. The devices with the structure La 0.67 Sr 0.33 MnO 3 (LSMO)/ organic spacer/ Co showed inverse magnetoresistance and low temperature operation with the vacuum evaporated πconjugated small molecule 8-hydroxyquinoline aluminium (Alq 3 ) while normal MR and high temperature operation was observed in devices with the πconjugated polymer poly (3-hexylthiophene). Due to the proximate values of the work functions of LSMO and Co with the highest occupied molecular orbital (HOMO) energy of the polymers, spin polarized carrier injection is much more efficient in the polymeric spin-valves compared to the organic spin-valves with Alq 3 where carrier injection is hindered due to greater barrier height. Efficient spin transport is also observed in polymeric spin-valves and can be attributed to longer conjugation in the polymeric chains compared to the small molecules.
Organic Electronics, 2005
We have observed bistability in single layer devices made from fullerenes (C60) mixed with polyst... more We have observed bistability in single layer devices made from fullerenes (C60) mixed with polystyrene (PS) and sandwiched between two Al electrodes. By merely changing the concentration of C60 in PS we found three distinctly different device properties, namely a true insulator, a bistable device switching between an OFF and an ON state having ON-OFF current ratios larger than 10 4 and a write-once, read-many times (WORM) device. An additional negative differential resistance (NDR) was observed in the ON state of both the bistable and WORM devices leading to multilevel switching capability of the devices. This opens up a wide range of application possibilities of such devices in disposable printable electronics.
Physical Review B, 2009
Magneto-electrical measurements were performed on diodes and bulk heterojunction solar cells (BHS... more Magneto-electrical measurements were performed on diodes and bulk heterojunction solar cells (BHSCs) to clarify the role of formation of coulombically bound electron-hole (e-h) pairs on the magnetoresistance (MR) response in organic thin film devices. BHSCs are suitable model systems because they effectively quench excitons but the probability of forming e-h pairs in them can be tuned over orders of magnitude by the choice of material and solvent in the blend. We have systematically varied the e-h recombination coefficients, which are directly proportional to the probability for the charge carriers to meet in space, and found that a reduced probability of electrons and holes meeting in space lead to disappearance of the MR. Our results clearly show that MR is a direct consequence of e-h pair formation. We also found that the MR line shape follows a power law-dependence of B 0.5 at higher fields.
We report on magnetotransport studies of regio regular poly (3-hexyl thiophene) (RRP3HT) based di... more We report on magnetotransport studies of regio regular poly (3-hexyl thiophene) (RRP3HT) based diodes and P3HT: 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]methanofullerene (PCBM) bulk heterojunction solar cells. While the P3HT diodes with non-magnetic electrodes ITO and Al show up to 16 % positive magnetoresistance (MR) under 300 mT magnetic field at room temperature, solar cells with much reduced electronhole pair (e-h) formation probability show almost negligible MR clearly suggesting that MR in organic semiconductors is strongly dependent on the probability of forming Coulombically bound e-h pairs. Voltage and temperature dependence of MR sign and magnitude is discussed for the diode devices which provides important insight of the scientifically puzzling yet technologically very promising magnetotransport properties of organic semiconductor based devices.
Nanotechnology, 2008
Organic memory using [6,6]-phenyl-C 61 butyric acid methyl ester: morphology, thickness and conce... more Organic memory using [6,6]-phenyl-C 61 butyric acid methyl ester: morphology, thickness and concentration dependence studies. Nanotechnology, volume 19, number 3, 035203.
Applied Physics Letters, 2008
A r i L a i h o , H i m a d r i S . M a j u m d a r , J a y a n t a K . B a r a l , F r e d r i k... more A r i L a i h o , H i m a d r i S . M a j u m d a r , J a y a n t a K . B a r a l , F r e d r i k J a n s s o n , R o n a l d Österbacka, and Olli Ikkala. 2008. Tuning the electrical switching of polymer/fullerene nanocomposite thin film devices by control of morphology. Applied Physics Letters, volume 93, number 20, 203309.
Thin Solid Films, 2003
We have fabricated and characterized sandwich-type devices based on an oriented polythiophene der... more We have fabricated and characterized sandwich-type devices based on an oriented polythiophene derivative. Depending on the voltage sweep direction, the devices showed the presence of two conducting states. Switching between the two states and an associated memory have earlier been studied by current-voltage characteristics. In this article, we have studied impedance spectroscopy of the material to model the device. We have studied conductance switching by measuring capacitance of the device, and have probed the high-and low-conducting states of the device. It has been shown that voltage variation of dielectric constant of the material depends on the voltage sweep direction. We have also demonstrated that capacitance of the device can probe the two states for data-storage applications. ᮊ
We report observation of ferromagnetic (FM) ordering in organic semiconductors, namely regioregul... more We report observation of ferromagnetic (FM) ordering in organic semiconductors, namely regioregular poly (3-hexyl thiophene) (RRP3HT) and 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]methanofullerene (PCBM), in the temperature range of 5 -300 K in addition to magnetoresistance (OMAR) observed in the diodes made from the same materials. Particle induced x-ray emission spectroscopy confirms the presence of dilute magnetic impurities in the materials mainly as residues from the synthesis process. However, upon blending these two materials with FM signal, the FM ordering is suppressed by a huge paramagnetic (PM) signal indicating ground state charge transfer formation in the blend. Together with the magneto-transport studies, these results indicate that OMAR response is observed in a device only when the corresponding active materials are FM. In the diodes with P3HT:PCBM complex, that as a blend shows PM response, OMAR vanishes almost completely. We propose that ferromagnetism in the active material can have important correlation with the OMAR response in the diodes.
... Sayani Majumdar (Abo Akademi University and University of Turku). ... to the formation of a t... more ... Sayani Majumdar (Abo Akademi University and University of Turku). ... to the formation of a thin spin-selective tunneling interface between LSMO and RRP3HT caused by the chemical bonding between RRP3HT and LSMO as observed by x-ray photoelectron spectroscopy ...
We report giant magnetoresistance up to 150% at low bias current and low temperature as well as r... more We report giant magnetoresistance up to 150% at low bias current and low temperature as well as room temperature magnetoresistance in polymeric spin-valves having the structure La 0.67 Sr 0.33 MnO 3 (LSMO)/conjugated polymer/Co. The conjugated polymersregiorandom and regioregular poly(3-hexyl thiophene) were used as the spacer materials.
Conjugated polymers have been used in data-storage devices. A ''state'' has been written by apply... more Conjugated polymers have been used in data-storage devices. A ''state'' has been written by applying a voltage pulse. The state of the device has been ''read'' from the current under a small probe voltage ͑0.2 V͒. The polymer retained the state for more than 1 h which can be refreshed or erased at will. The stored space charges under a voltage pulse have been found to control the charge injection and hence the device current. Their slow relaxation process has resulted in the use of conjugated polymers in memory device applications. Hysteresis-type behavior has been observed in the current-voltage characteristics. The density of stored charges at the polymer layer near the metal/polymer interface has been found to depend on the voltage amplitude. The relaxation of the stored charges has been studied by applying two voltage pulses. By varying the delay between the two pulses, during which the space charges relax or redistribute, the time constant for charge relaxation has been calculated. The time constant was found to be independent of the density of the space charges or of the pace at which they were stored.
Journal of Materials Chemistry, 2006
In this paper, we present a new method that combines surface modification of indium-tin oxide (IT... more In this paper, we present a new method that combines surface modification of indium-tin oxide (ITO) and electropolymerization to prepare thin, sulfonated poly(aniline) (SPAN) films with good surface coverage. The surface modification enhances the growth of the SPAN film resulting in a better rectification signal than for SPAN films polymerized on unmodified ITO substrates. The films were characterized by FTIR, scanning electron microscopy (SEM) and atomic force microscopy (AFM). The sulfonation degree of SPAN was determined to 29% by X-ray photoelectron spectroscopy (XPS). UV-VIS spectroscopy shows that the pH sensitivity of SPAN is suppressed due to sulfonation of the polymer backbone. It is also shown that conversion of the SPAN film to the emeraldine salt (ES) form after polymerization is crucial for obtaining a high rectification signal. This is an important practical aspect in the preparation procedure of organic electronic devices.