Tianlong Wen | Carnegie Mellon University (original) (raw)

Papers by Tianlong Wen

Large area dense hole arrays with a feature size of 10 nm were generated using self-assembled mon... more Large area dense hole arrays with a feature size of 10 nm were generated using self-assembled monolayers of nanoparticles as etch masks. To fabricate the hole arrays, monolayers of nanoparticles were irradiated by electron beam to turn surfactants into amorphous carbon, treated by acid to remove the nanoparticle cores, and then etched by CF4 to deepen the holes. Evaporated gold films preferentially diffuse into the holes to generate gold nanoparticle arrays. However no obvious diffusion into holes was observed for a sputtered iron platinum film.

Applied Physics Letters, Jan 1, 2012

Unusual positive exchange bias found in Fe/CoOx nanoparticle bilayer films is correlated to a cha... more Unusual positive exchange bias found in Fe/CoOx nanoparticle bilayer films is correlated to a characteristic magnetic spin-glass (SG) in CoOx, with the SG magnetization coupled antiparallel with the Fe magnetization upon field cooling. This SG magnetization has strong field- and time-dependence which displays unusual upward magnetic relaxation behavior in thermoremanent magnetization measurements. The antiparallel coupling is shown to result predominantly from the antiferromagnetic superexchange of the Fe2+-O-Co3+ couple, of the oxygen-terminated CoOx at the interface. These experimental results reveal the possibility of manipulating the exchange bias effect via an indirect exchange coupling mechanism.

ACS nano, Jan 1, 2011

Large-area self-assembled monolayers of nanoparticles are fabricated on the surface of deionized ... more Large-area self-assembled monolayers of nanoparticles are fabricated on the surface of deionized water by controlled evaporation of nanoparticles dispersed in a binary solvent mixture. The difference in solvent volatility and partial coverage of the trough leads to a flux of nanoparticles toward the evaporation front. The monolayers are comprised of monodisperse magnetite and gold nanoparticles or slightly more polydisperse manganese oxide nanoparticles. The floating monolayers are transferred onto different substrates by the Langmuir–Schaefer method. Surfactants in the colloidal solution and substrate materials have significant impact on the monolayer formation. Bilayers of nanoparticles with different twist angles between layers are also obtained by double deposition.

This Perspective describes recent progress in the development of functional magnetic nanoparticle... more This Perspective describes recent progress in the development of functional magnetic nanoparticle assemblies. After describing the formation of two- and three-dimensional particle arrays in terms of the size-dependent driving forces, we focus on magnetic nanoparticle arrays. We discuss how the self-organized structure can modify the magnetic behavior, relative to that of isolated particles. We highlight an important development, described in this issue of ACS Nano by Kostiainen and co-workers, who have demonstrated not only the novel aqueous self-assembly of magnetic particles but also controlled and reversible disassembly. Finally, we explore two inter-related future directions for self-assembly of magnetic nanoparticles: the formation of more complex, hierarchical structures and the integration of self-assembly with fabrication techniques for electronic devices.

ChemInform, Jan 1, 2012

Recently, magnetic nanocomposites (MNC) have aroused significant scientific and technological int... more Recently, magnetic nanocomposites (MNC) have aroused significant scientific and technological interests because their properties strongly rely on the interplay between those of the constituent components. Here, using three types of cobalt-based MNCs, we demonstrate how their physical behaviour, including thermal, electrical and magnetic, can be strongly affected by such interplays. First, using Au core -Co shell nanoparticles (NPs), we demonstrate that their thermal stabilities are critically dependent on various boundaries and they structurally transform from the core-shells to the peanut structures via several intermediate states by a series of energy minimizations including the grain boundaries, Co/Au interface and strain. Second, the microstructures of the MNC are co-determined by the properties of the individual components, which in turn will strongly affect their overall properties. We illustrate this by a careful study of the electron transport in cobalt/poly (3-hexylthiophene, 2, 5-diyl) (P3HT) hybrid thin films, and show that they satisfy a fluctuation-induced tunnelling model that is strongly depended on their microstructures; moreover, a magnetoresistance in these thin films was also observed. Finally, the magnetic properties and phase stability of MNCs can also be strongly altered as a result of this interplay. Three phase transformations are observed in cobalt ferrofluids for T ∼ 10-300 K, namely second order magnetic phase transformations (blocked-unblocked transition) at the blocking temperature of the magnetic NP, first order magnetic and structural phase transformations at the solvent melting temperature, T M , and second order premelting transformation at T PM < T < T M . These transformations show specific magnetic signatures in field-cool and zero-field-cool magnetization measurements and are qualitatively in agreement with predictions using M-spectrum theory.

Journal of Physics D: Applied Physics, Jan 1, 2011

Recently, magnetic nanocomposites (MNC) have aroused significant scientific and technological int... more Recently, magnetic nanocomposites (MNC) have aroused significant scientific and technological interests because their properties strongly rely on the interplay between those of the constituent components. Here, using three types of cobalt-based MNCs, we demonstrate how their physical behaviour, including thermal, electrical and magnetic, can be strongly affected by such interplays. First, using Aucore–Coshell nanoparticles (NPs), we demonstrate that their thermal stabilities are critically dependent on various boundaries and they structurally transform from the core–shells to the peanut structures via several intermediate states by a series of energy minimizations including the grain boundaries, Co/Au interface and strain. Second, the microstructures of the MNC are co-determined by the properties of the individual components, which in turn will strongly affect their overall properties. We illustrate this by a careful study of the electron transport in cobalt/poly (3-hexylthiophene, 2, 5-diyl) (P3HT) hybrid thin films, and show that they satisfy a fluctuation-induced tunnelling model that is strongly depended on their microstructures; moreover, a magnetoresistance in these thin films was also observed. Finally, the magnetic properties and phase stability of MNCs can also be strongly altered as a result of this interplay. Three phase transformations are observed in cobalt ferrofluids for T ~ 10–300 K, namely second order magnetic phase transformations (blocked–unblocked transition) at the blocking temperature of the magnetic NP, first order magnetic and structural phase transformations at the solvent melting temperature, TM, and second order premelting transformation at TPM < T < TM. These transformations show specific magnetic signatures in field-cool and zero-field-cool magnetization measurements and are qualitatively in agreement with predictions using M-spectrum theory.

Journal of Applied Physics, Jan 1, 2010

Zero-field-cooling and field-cooling (FC) measurements were performed on ferrofluids of cobalt ma... more Zero-field-cooling and field-cooling (FC) measurements were performed on ferrofluids of cobalt magnetic nanoparticles (MNPs) in various organic solvent. Two peaks, one broad peak corresponding to the blocking transition (TB), and one sharp peak corresponding to the melting of the solvent (TM), were observed. Furthermore, for a given MNP size, when the blocking and melting transitions were superposed by choosing an appropriate solvent, the strongest intensity of the sharp peak at the melting point of the organic solvent was obtained. This observation is explained by applying the M spectrum theory. Additionally, a first order, melting-induced magnetic phase transformation was observed at the melting point of the solvent. Associated with the first order phase transition and the supercooling effect, a thermal hysteresis loop in the FC curve was observed.

Journal of Applied Physics, Jan 1, 2011

Aucore-Coshell nanoparticles were synthesized by a wet-chemical method, and their magnetic proper... more Aucore-Coshell nanoparticles were synthesized by a wet-chemical method, and their magnetic properties were characterized. By comparing the blocking temperature of Aucore-Coshell nanoparticles to pure cobalt nanoparticles of the same size, it is proposed that cobalt spins in the shell are all aligned by exchange interaction to form single domain nano shells. At the same time, a demagnetizing field in the gold core develops and is aligned along the magnetization direction of the cobalt shell. A greatly enhanced coercivity of Aucore-Coshell nanoparticles at 10 K, compared to the pure cobalt sample, was observed. We propose that the enhanced coercivity of Aucore-Coshell nanoparticles is due to the pinning effect of cobalt spins at the Au/Co interface. The pinning mechanisms include strain pinning and demagnetizing field pinning.

Microscopy and …, Jan 1, 2011

Nanoparticles have attracted considerable attention due to their unique properties. Below a certa... more Nanoparticles have attracted considerable attention due to their unique properties. Below a certain size, around 100 nm, nanoparticles show different chemical and physical properties from bulk materials and atoms. Such properties depend on size, shape, structure and morphology and thus it is not only necessary to develop robust methods to synthesize nanoparticles but also characterize them. Normally, a monodispersed size distribution or a given particle shape are necessary for application of nanoparticles in diverse fields such as biology or energy production. In the present case, immiscible binary Co-Au core shell nanoparticles, with dual magnetic optical functionality and potential in biological application are investigated. Their sizes and shapes as a function of diverse parameters of synthesis is important as well as basic nucleation and growth processes of both core and shell (CS).

The Journal of Physical Chemistry C, Jan 1, 2010

Aucore−Coshell nanoparticles were synthesized by a wet-chemical method in organic solvent and the... more Aucore−Coshell nanoparticles were synthesized by a wet-chemical method in organic solvent and their thermal stability was studied by annealing experiments over a wide temperature range (200−450 °C). During heating, atoms within the core−shell nanoparticles migrate to transform their initial core−shell morphology to a stable “peanut” structure. We show that this morphological transformation occurs by a well-defined series of energy minimization steps that sequentially reduce the energy due to intraphase grain boundaries within the cobalt shell, the interface energy between the gold core and the cobalt shell, and the overall strain energy.

Applied Physics …, Jan 1, 2009

Cobalt/poly (3-hexylthiophene, 2, 5-diyl) (P3HT) hybrid thin films were prepared by a wet chemica... more Cobalt/poly (3-hexylthiophene, 2, 5-diyl) (P3HT) hybrid thin films were prepared by a wet chemical method. Their microstructure consists of a nanoscale mixture of a crystalline P3HT matrix, interspersed with amorphous P3HT regions containing the cobalt nanoparticles. Magnetic and transport measurements are consistent with this microstructure and the temperature dependence of the resistance of these hybrid systems is well-fitted (R2 = 0.9993) to the fluctuation induced tunneling model. Moreover, by applying a magnetic field, a magnetoresistance ratio of 3% was observed in 17 vol % Co hybrid films at 10 K.

Journal of sol-gel …, Jan 1, 2005

This paper reports template-based growth of nanorod arrays by combination of sol–gel processing a... more This paper reports template-based growth of nanorod arrays by combination of sol–gel processing and
centrifugation. The technical concept is simple and straightforward; centrifugation force drives the sol nanoclusters
into the pores of the template, filling the pores completely to form nanorods. However, simulation reveals the
formation of nanorods inside pores is more complex; centrifugation force is insufficient to overcome the energy
barrier that prevents agglomeration of two nanoclusters. Thermal fluctuation and solvent evaporation upon drying
are considered to be the forces leading to the eventual agglomeration of concentrated nanoclusters inside template
pores. Dense nanorods with ∼200 nm diameter and ∼10 µm length were readily obtained from polymeric SiO2,
colloidal SiO2, TiO2 and PZT sols after heat treatment.

Large area dense hole arrays with a feature size of 10 nm were generated using self-assembled mon... more Large area dense hole arrays with a feature size of 10 nm were generated using self-assembled monolayers of nanoparticles as etch masks. To fabricate the hole arrays, monolayers of nanoparticles were irradiated by electron beam to turn surfactants into amorphous carbon, treated by acid to remove the nanoparticle cores, and then etched by CF4 to deepen the holes. Evaporated gold films preferentially diffuse into the holes to generate gold nanoparticle arrays. However no obvious diffusion into holes was observed for a sputtered iron platinum film.

Applied Physics Letters, Jan 1, 2012

Unusual positive exchange bias found in Fe/CoOx nanoparticle bilayer films is correlated to a cha... more Unusual positive exchange bias found in Fe/CoOx nanoparticle bilayer films is correlated to a characteristic magnetic spin-glass (SG) in CoOx, with the SG magnetization coupled antiparallel with the Fe magnetization upon field cooling. This SG magnetization has strong field- and time-dependence which displays unusual upward magnetic relaxation behavior in thermoremanent magnetization measurements. The antiparallel coupling is shown to result predominantly from the antiferromagnetic superexchange of the Fe2+-O-Co3+ couple, of the oxygen-terminated CoOx at the interface. These experimental results reveal the possibility of manipulating the exchange bias effect via an indirect exchange coupling mechanism.

ACS nano, Jan 1, 2011

Large-area self-assembled monolayers of nanoparticles are fabricated on the surface of deionized ... more Large-area self-assembled monolayers of nanoparticles are fabricated on the surface of deionized water by controlled evaporation of nanoparticles dispersed in a binary solvent mixture. The difference in solvent volatility and partial coverage of the trough leads to a flux of nanoparticles toward the evaporation front. The monolayers are comprised of monodisperse magnetite and gold nanoparticles or slightly more polydisperse manganese oxide nanoparticles. The floating monolayers are transferred onto different substrates by the Langmuir–Schaefer method. Surfactants in the colloidal solution and substrate materials have significant impact on the monolayer formation. Bilayers of nanoparticles with different twist angles between layers are also obtained by double deposition.

This Perspective describes recent progress in the development of functional magnetic nanoparticle... more This Perspective describes recent progress in the development of functional magnetic nanoparticle assemblies. After describing the formation of two- and three-dimensional particle arrays in terms of the size-dependent driving forces, we focus on magnetic nanoparticle arrays. We discuss how the self-organized structure can modify the magnetic behavior, relative to that of isolated particles. We highlight an important development, described in this issue of ACS Nano by Kostiainen and co-workers, who have demonstrated not only the novel aqueous self-assembly of magnetic particles but also controlled and reversible disassembly. Finally, we explore two inter-related future directions for self-assembly of magnetic nanoparticles: the formation of more complex, hierarchical structures and the integration of self-assembly with fabrication techniques for electronic devices.

ChemInform, Jan 1, 2012

Recently, magnetic nanocomposites (MNC) have aroused significant scientific and technological int... more Recently, magnetic nanocomposites (MNC) have aroused significant scientific and technological interests because their properties strongly rely on the interplay between those of the constituent components. Here, using three types of cobalt-based MNCs, we demonstrate how their physical behaviour, including thermal, electrical and magnetic, can be strongly affected by such interplays. First, using Au core -Co shell nanoparticles (NPs), we demonstrate that their thermal stabilities are critically dependent on various boundaries and they structurally transform from the core-shells to the peanut structures via several intermediate states by a series of energy minimizations including the grain boundaries, Co/Au interface and strain. Second, the microstructures of the MNC are co-determined by the properties of the individual components, which in turn will strongly affect their overall properties. We illustrate this by a careful study of the electron transport in cobalt/poly (3-hexylthiophene, 2, 5-diyl) (P3HT) hybrid thin films, and show that they satisfy a fluctuation-induced tunnelling model that is strongly depended on their microstructures; moreover, a magnetoresistance in these thin films was also observed. Finally, the magnetic properties and phase stability of MNCs can also be strongly altered as a result of this interplay. Three phase transformations are observed in cobalt ferrofluids for T ∼ 10-300 K, namely second order magnetic phase transformations (blocked-unblocked transition) at the blocking temperature of the magnetic NP, first order magnetic and structural phase transformations at the solvent melting temperature, T M , and second order premelting transformation at T PM < T < T M . These transformations show specific magnetic signatures in field-cool and zero-field-cool magnetization measurements and are qualitatively in agreement with predictions using M-spectrum theory.

Journal of Physics D: Applied Physics, Jan 1, 2011

Recently, magnetic nanocomposites (MNC) have aroused significant scientific and technological int... more Recently, magnetic nanocomposites (MNC) have aroused significant scientific and technological interests because their properties strongly rely on the interplay between those of the constituent components. Here, using three types of cobalt-based MNCs, we demonstrate how their physical behaviour, including thermal, electrical and magnetic, can be strongly affected by such interplays. First, using Aucore–Coshell nanoparticles (NPs), we demonstrate that their thermal stabilities are critically dependent on various boundaries and they structurally transform from the core–shells to the peanut structures via several intermediate states by a series of energy minimizations including the grain boundaries, Co/Au interface and strain. Second, the microstructures of the MNC are co-determined by the properties of the individual components, which in turn will strongly affect their overall properties. We illustrate this by a careful study of the electron transport in cobalt/poly (3-hexylthiophene, 2, 5-diyl) (P3HT) hybrid thin films, and show that they satisfy a fluctuation-induced tunnelling model that is strongly depended on their microstructures; moreover, a magnetoresistance in these thin films was also observed. Finally, the magnetic properties and phase stability of MNCs can also be strongly altered as a result of this interplay. Three phase transformations are observed in cobalt ferrofluids for T ~ 10–300 K, namely second order magnetic phase transformations (blocked–unblocked transition) at the blocking temperature of the magnetic NP, first order magnetic and structural phase transformations at the solvent melting temperature, TM, and second order premelting transformation at TPM < T < TM. These transformations show specific magnetic signatures in field-cool and zero-field-cool magnetization measurements and are qualitatively in agreement with predictions using M-spectrum theory.

Journal of Applied Physics, Jan 1, 2010

Zero-field-cooling and field-cooling (FC) measurements were performed on ferrofluids of cobalt ma... more Zero-field-cooling and field-cooling (FC) measurements were performed on ferrofluids of cobalt magnetic nanoparticles (MNPs) in various organic solvent. Two peaks, one broad peak corresponding to the blocking transition (TB), and one sharp peak corresponding to the melting of the solvent (TM), were observed. Furthermore, for a given MNP size, when the blocking and melting transitions were superposed by choosing an appropriate solvent, the strongest intensity of the sharp peak at the melting point of the organic solvent was obtained. This observation is explained by applying the M spectrum theory. Additionally, a first order, melting-induced magnetic phase transformation was observed at the melting point of the solvent. Associated with the first order phase transition and the supercooling effect, a thermal hysteresis loop in the FC curve was observed.

Journal of Applied Physics, Jan 1, 2011

Aucore-Coshell nanoparticles were synthesized by a wet-chemical method, and their magnetic proper... more Aucore-Coshell nanoparticles were synthesized by a wet-chemical method, and their magnetic properties were characterized. By comparing the blocking temperature of Aucore-Coshell nanoparticles to pure cobalt nanoparticles of the same size, it is proposed that cobalt spins in the shell are all aligned by exchange interaction to form single domain nano shells. At the same time, a demagnetizing field in the gold core develops and is aligned along the magnetization direction of the cobalt shell. A greatly enhanced coercivity of Aucore-Coshell nanoparticles at 10 K, compared to the pure cobalt sample, was observed. We propose that the enhanced coercivity of Aucore-Coshell nanoparticles is due to the pinning effect of cobalt spins at the Au/Co interface. The pinning mechanisms include strain pinning and demagnetizing field pinning.

Microscopy and …, Jan 1, 2011

Nanoparticles have attracted considerable attention due to their unique properties. Below a certa... more Nanoparticles have attracted considerable attention due to their unique properties. Below a certain size, around 100 nm, nanoparticles show different chemical and physical properties from bulk materials and atoms. Such properties depend on size, shape, structure and morphology and thus it is not only necessary to develop robust methods to synthesize nanoparticles but also characterize them. Normally, a monodispersed size distribution or a given particle shape are necessary for application of nanoparticles in diverse fields such as biology or energy production. In the present case, immiscible binary Co-Au core shell nanoparticles, with dual magnetic optical functionality and potential in biological application are investigated. Their sizes and shapes as a function of diverse parameters of synthesis is important as well as basic nucleation and growth processes of both core and shell (CS).

The Journal of Physical Chemistry C, Jan 1, 2010

Aucore−Coshell nanoparticles were synthesized by a wet-chemical method in organic solvent and the... more Aucore−Coshell nanoparticles were synthesized by a wet-chemical method in organic solvent and their thermal stability was studied by annealing experiments over a wide temperature range (200−450 °C). During heating, atoms within the core−shell nanoparticles migrate to transform their initial core−shell morphology to a stable “peanut” structure. We show that this morphological transformation occurs by a well-defined series of energy minimization steps that sequentially reduce the energy due to intraphase grain boundaries within the cobalt shell, the interface energy between the gold core and the cobalt shell, and the overall strain energy.

Applied Physics …, Jan 1, 2009

Cobalt/poly (3-hexylthiophene, 2, 5-diyl) (P3HT) hybrid thin films were prepared by a wet chemica... more Cobalt/poly (3-hexylthiophene, 2, 5-diyl) (P3HT) hybrid thin films were prepared by a wet chemical method. Their microstructure consists of a nanoscale mixture of a crystalline P3HT matrix, interspersed with amorphous P3HT regions containing the cobalt nanoparticles. Magnetic and transport measurements are consistent with this microstructure and the temperature dependence of the resistance of these hybrid systems is well-fitted (R2 = 0.9993) to the fluctuation induced tunneling model. Moreover, by applying a magnetic field, a magnetoresistance ratio of 3% was observed in 17 vol % Co hybrid films at 10 K.

Journal of sol-gel …, Jan 1, 2005

This paper reports template-based growth of nanorod arrays by combination of sol–gel processing a... more This paper reports template-based growth of nanorod arrays by combination of sol–gel processing and
centrifugation. The technical concept is simple and straightforward; centrifugation force drives the sol nanoclusters
into the pores of the template, filling the pores completely to form nanorods. However, simulation reveals the
formation of nanorods inside pores is more complex; centrifugation force is insufficient to overcome the energy
barrier that prevents agglomeration of two nanoclusters. Thermal fluctuation and solvent evaporation upon drying
are considered to be the forces leading to the eventual agglomeration of concentrated nanoclusters inside template
pores. Dense nanorods with ∼200 nm diameter and ∼10 µm length were readily obtained from polymeric SiO2,
colloidal SiO2, TiO2 and PZT sols after heat treatment.