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Ferron T, Fiori ME, Ediger MD, DeLongchamp DM, Sunday DF. Composition Dictates Molecular Orientation at the Heterointerfaces of Vapor-Deposited Glasses. JACS AU 2023; 3:1931-1938. [PMID: 37502150 PMCID: PMC10369407 DOI: 10.1021/jacsau.3c00168] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 04/06/2023] [Revised: 05/22/2023] [Accepted: 06/09/2023] [Indexed: 07/29/2023]
Abstract
Physical vapor deposition (PVD) can prepare organic glasses with a preferred molecular orientation. The relationships between deposition conditions and orientation have been extensively investigated in the film bulk. The role of interfaces on the structure is less well understood and remains a key knowledge gap, as the interfacial region can govern glass stability and optoelectronic properties. Robust experimental characterization has remained elusive due to complexities in interrogating molecular organization in amorphous, organic materials. Polarized soft X-rays are sensitive to both the composition and the orientation of transition dipole moments in the film, making them uniquely suited to probe molecular orientation in amorphous soft matter. Here, we utilize polarized resonant soft X-ray reflectivity (P-RSoXR) to simultaneously depth profile the composition and molecular orientation of a bilayer prepared through the physical vapor deposition of 1,4-di-[4-(N,N-diphenyl)amino]styryl-benzene (DSA-Ph) on a film of aluminum-tris(8-hydroxyquinoline) (Alq3). The bulk orientation of the DSA-Ph layer is controlled by varying deposition conditions. Utilizing P-RSoXR to depth profile the films enables determination of both the bulk orientation of DSA-Ph and the orientation near the Alq3 interface. At the Alq3 surface, DSA-Ph always lies with its long axis parallel to the interface, before transitioning into the bulk orientation. This is likely due to the lower mobility and higher glass transition of Alq3, as the first several monolayers of DSA-Ph deposited on Alq3 appear to behave as a blend. We further show how orientation at the interface correlates with the bulk behavior of a codeposited glass of similar blend composition, demonstrating a straightforward approach to predicting molecular orientation at heterointerfaces. This work provides key insights into how molecules orient during vapor deposition and offers methods to predict this property, a critical step toward controlling interfacial behavior in soft matter.
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Affiliation(s)
- Thomas
J. Ferron
- National
Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States
| | - Marie E. Fiori
- Department
of Chemistry, University of Wisconsin−Madison, Madison, Wisconsin 53706, United States
| | - M. D. Ediger
- Department
of Chemistry, University of Wisconsin−Madison, Madison, Wisconsin 53706, United States
| | - Dean M. DeLongchamp
- National
Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States
| | - Daniel F. Sunday
- National
Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States
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2
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Cheng S, Lee Y, Yu J, Yu L, Ediger MD. Surface Equilibration Mechanism Controls the Stability of a Model Codeposited Glass Mixture of Organic Semiconductors. J Phys Chem Lett 2023; 14:4297-4303. [PMID: 37129465 DOI: 10.1021/acs.jpclett.3c00728] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/03/2023]
Abstract
While previous work has identified the conditions for preparing ultrastable single-component organic glasses by physical vapor deposition (PVD), little is known about the stability of codeposited mixtures. Here, we prepared binary PVD glasses of organic semiconductors, TPD (N,N'-Bis(3-methylphenyl)-N,N'-diphenylbenzidine) and m-MTDATA (4,4',4″-Tris[phenyl(m-tolyl)amino]triphenylamine), with a 50:50 mass concentration over a wide range of substrate temperatures (Tsub). The enthalpy and kinetic stability are evaluated with differential scanning calorimetry and spectroscopic ellipsometry. Binary organic semiconductor glasses with exceptional thermodynamic and kinetic stability comparable to the most stable single-component organic glasses are obtained when deposited at Tsub = 0.78-0.90Tg (where Tg is the conventional glass transition temperature). When deposited at 0.94Tg, the enthalpy of the m-MTDATA/TPD glass equals that expected for the equilibrium liquid at that temperature. Thus, the surface equilibration mechanism previously advanced for single-component PVD glasses is also applicable for these codeposited glasses. These results provide an avenue for designing high-performance organic electronic devices.
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Affiliation(s)
- Shinian Cheng
- Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States
| | - Yejung Lee
- Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States
| | - Junguang Yu
- School of Pharmacy, University of Wisconsin-Madison, Madison, Wisconsin 53705, United States
| | - Lian Yu
- School of Pharmacy, University of Wisconsin-Madison, Madison, Wisconsin 53705, United States
| | - M D Ediger
- Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States
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3
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Tanaka M, Auffray M, Nakanotani H, Adachi C. Spontaneous formation of metastable orientation with well-organized permanent dipole moment in organic glassy films. NATURE MATERIALS 2022; 21:819-825. [PMID: 35637340 DOI: 10.1038/s41563-022-01265-7] [Citation(s) in RCA: 9] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/21/2021] [Accepted: 04/20/2022] [Indexed: 06/15/2023]
Abstract
The performance of organic optoelectronic and energy-harvesting devices is largely determined by the molecular orientation and resultant permanent dipole moment, yet this property is difficult to control during film preparation. Here, we demonstrate the active control of dipole direction-that is, vector direction and magnitude-in organic glassy films by physical vapour deposition. An organic glassy film with metastable permanent dipole moment orientation can be obtained by utilizing the small surface free energy of a trifluoromethyl unit and intramolecular permanent dipole moment induced by functional groups. The proposed molecular design rule could pave a way toward the formation of spontaneously polarized organic glassy films, leading to improvement in the performance of organic molecular devices.
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Affiliation(s)
- Masaki Tanaka
- Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, Nishi-ku, Fukuoka, Japan.
- Department of Biotechnology and Life Science, Tokyo University of Agriculture and Technology, Tokyo, Japan.
| | - Morgan Auffray
- Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, Nishi-ku, Fukuoka, Japan
| | - Hajime Nakanotani
- Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, Nishi-ku, Fukuoka, Japan.
- International Institute for Carbon Neutral Energy Research (I2CNER), Kyushu University, Nishi-ku, Fukuoka, Japan.
| | - Chihaya Adachi
- Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, Nishi-ku, Fukuoka, Japan.
- International Institute for Carbon Neutral Energy Research (I2CNER), Kyushu University, Nishi-ku, Fukuoka, Japan.
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4
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Wang S, Zhu Z, Shi C, Li S, Liu Y, Zhang D, Wang Q, Zhao L, Wang W, Dong X. Enhanced performance of solution-processed OLEDs by altering the molecular transition dipole moment orientation of emission layers. SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2022; 271:120933. [PMID: 35104742 DOI: 10.1016/j.saa.2022.120933] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/17/2021] [Revised: 12/17/2021] [Accepted: 01/17/2022] [Indexed: 06/14/2023]
Abstract
Transition dipole moment orientation is one of the crucial factors, which can enhance the performance of organic light-emitting diodes (OLEDs). In this work, the transition dipole moment orientation of the host-guest emission layers (EMLs) prepared by solution method annealed at different temperatures were systematically studied by analyzing the angle-dependent PL spectrum. When the EMLs of 2DPAc-MPM (20 wt%): DPEPO was annealed at the temperature of 115 °C, the photoluminescence quantum yield (PLQY) was enhanced to 78%±3.5%, and the vertical dipole ratio was reduced to 0.34. The lower vertical orientation of transition dipoles is conducted to improve the electron and hole mobility, which was testified by the hole and electron only devices. The lower vertical dipole ratio, higher PLQY and carrier mobility together enhanced the performance of solution processed OLEDs.
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Affiliation(s)
- Shuai Wang
- School of Physical Science and Information Technology, Liaocheng University, Shandong 252059, China; Shandong Provincial Key Laboratory of Optical Communication Science and Technology, Shandong 252059, China
| | - Zhongchang Zhu
- School of Physical Science and Information Technology, Liaocheng University, Shandong 252059, China; Shandong Provincial Key Laboratory of Optical Communication Science and Technology, Shandong 252059, China
| | - Chaojun Shi
- School of Physical Science and Information Technology, Liaocheng University, Shandong 252059, China; Shandong Provincial Key Laboratory of Optical Communication Science and Technology, Shandong 252059, China
| | - Shuhong Li
- School of Physical Science and Information Technology, Liaocheng University, Shandong 252059, China; Shandong Provincial Key Laboratory of Optical Communication Science and Technology, Shandong 252059, China.
| | - Yunlong Liu
- School of Physical Science and Information Technology, Liaocheng University, Shandong 252059, China; Shandong Provincial Key Laboratory of Optical Communication Science and Technology, Shandong 252059, China
| | - Dong Zhang
- School of Physical Science and Information Technology, Liaocheng University, Shandong 252059, China; Shandong Provincial Key Laboratory of Optical Communication Science and Technology, Shandong 252059, China
| | - Qingru Wang
- School of Physical Science and Information Technology, Liaocheng University, Shandong 252059, China; Shandong Provincial Key Laboratory of Optical Communication Science and Technology, Shandong 252059, China
| | - Ling Zhao
- School of Physical Science and Information Technology, Liaocheng University, Shandong 252059, China; Shandong Provincial Key Laboratory of Optical Communication Science and Technology, Shandong 252059, China
| | - Wenjun Wang
- School of Physical Science and Information Technology, Liaocheng University, Shandong 252059, China; Shandong Provincial Key Laboratory of Optical Communication Science and Technology, Shandong 252059, China.
| | - Xiaochen Dong
- School of Physical Science and Information Technology, Liaocheng University, Shandong 252059, China; Shandong Provincial Key Laboratory of Optical Communication Science and Technology, Shandong 252059, China; Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), School of Physical and Mathematical Sciences, Nanjing Tech University (NanjingTech), Nanjing 211800, China
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5
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Surface equilibration mechanism controls the molecular packing of glassy molecular semiconductors at organic interfaces. Proc Natl Acad Sci U S A 2021; 118:2111988118. [PMID: 34645709 DOI: 10.1073/pnas.2111988118] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 09/07/2021] [Indexed: 12/31/2022] Open
Abstract
Glasses prepared by physical vapor deposition (PVD) are anisotropic, and the average molecular orientation can be varied significantly by controlling the deposition conditions. While previous work has characterized the average structure of thick PVD glasses, most experiments are not sensitive to the structure near an underlying substrate or interface. Given the profound influence of the substrate on the growth of crystalline or liquid crystalline materials, an underlying substrate might be expected to substantially alter the structure of a PVD glass, and this near-interface structure is important for the function of organic electronic devices prepared by PVD, such as organic light-emitting diodes. To study molecular packing near buried organic-organic interfaces, we prepare superlattice structures (stacks of 5- or 10-nm layers) of organic semiconductors, Alq3 (Tris-(8-hydroxyquinoline)aluminum) and DSA-Ph (1,4-di-[4-(N,N-diphenyl)amino]styrylbenzene), using PVD. Superlattice structures significantly increase the fraction of the films near buried interfaces, thereby allowing for quantitative characterization of interfacial packing. Remarkably, both X-ray scattering and spectroscopic ellipsometry indicate that the substrate exerts a negligible influence on PVD glass structure. Thus, the surface equilibration mechanism previously advanced for thick films can successfully describe PVD glass structure even within the first monolayer of deposition on an organic substrate.
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6
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Tenopala-Carmona F, Lee OS, Crovini E, Neferu AM, Murawski C, Olivier Y, Zysman-Colman E, Gather MC. Identification of the Key Parameters for Horizontal Transition Dipole Orientation in Fluorescent and TADF Organic Light-Emitting Diodes. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2021; 33:e2100677. [PMID: 34338351 DOI: 10.1002/adma.202100677] [Citation(s) in RCA: 38] [Impact Index Per Article: 12.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/26/2021] [Revised: 04/21/2021] [Indexed: 06/13/2023]
Abstract
In organic light-emitting diodes (OLEDs), horizontal orientation of the emissive transition dipole moment (TDM) can improve light outcoupling efficiency by up to 50% relative to random orientation. Therefore, there have been extensive efforts to identify drivers of horizontal orientation. The aspect ratio of the emitter molecule and the glass-transition temperature (Tg ) of the films are currently regarded as particularly important. However, there remains a paucity of systematic studies that establish the extent to which these and other parameters control orientation in the wide range of emitter systems relevant for state-of-the-art OLEDs. Here, recent work on molecular orientation of fluorescent and thermally activated delayed fluorescent emitters in vacuum-processed OLEDs is reviewed. Additionally, to identify parameters linked to TDM orientation, a meta-analysis of 203 published emitter systems is conducted and combined with density-functional theory calculations. Molecular weight (MW) and linearity are identified as key parameters in neat systems. In host-guest systems with low-MW emitters, orientation is mostly influenced by the host Tg , whereas the length and MW of the emitter become more relevant for systems involving higher-MW emitters. To close, a perspective of where the field must advance to establish a comprehensive model of molecular orientation is given.
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Affiliation(s)
- Francisco Tenopala-Carmona
- Organic Semiconductor Centre, SUPA School of Physics and Astronomy, University of St Andrews, St Andrews, KY16 9SS, UK
- Humboldt Centre for Nano- and Biophotonics, Department of Chemistry, University of Cologne, Greinstr. 4-6, 50939, Köln, Germany
| | - Oliver S Lee
- Organic Semiconductor Centre, SUPA School of Physics and Astronomy, University of St Andrews, St Andrews, KY16 9SS, UK
- Organic Semiconductor Centre, EaStCHEM School of Chemistry, University of St Andrews, St Andrews, KY16 9ST, UK
| | - Ettore Crovini
- Organic Semiconductor Centre, EaStCHEM School of Chemistry, University of St Andrews, St Andrews, KY16 9ST, UK
| | - Ana M Neferu
- Organic Semiconductor Centre, SUPA School of Physics and Astronomy, University of St Andrews, St Andrews, KY16 9SS, UK
- Organic Semiconductor Centre, EaStCHEM School of Chemistry, University of St Andrews, St Andrews, KY16 9ST, UK
| | - Caroline Murawski
- Organic Semiconductor Centre, SUPA School of Physics and Astronomy, University of St Andrews, St Andrews, KY16 9SS, UK
| | - Yoann Olivier
- Unité de Chimie Physique Théorique et Structurale & Laboratoire de Physique du Solide, Namur Institute of Structured Matter, Université de Namur, Rue de Bruxelles, 61, Namur, 5000, Belgium
| | - Eli Zysman-Colman
- Organic Semiconductor Centre, EaStCHEM School of Chemistry, University of St Andrews, St Andrews, KY16 9ST, UK
| | - Malte C Gather
- Organic Semiconductor Centre, SUPA School of Physics and Astronomy, University of St Andrews, St Andrews, KY16 9SS, UK
- Humboldt Centre for Nano- and Biophotonics, Department of Chemistry, University of Cologne, Greinstr. 4-6, 50939, Köln, Germany
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7
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Cui D, Wang S, Li S, Liu Y, Gao X, Wang W, Dong X. Improving the performance of OLEDs by controlling the molecular orientation in charge carrier transport layers. OPTICS EXPRESS 2021; 29:16845-16856. [PMID: 34154237 DOI: 10.1364/oe.418566] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/06/2021] [Accepted: 05/04/2021] [Indexed: 06/13/2023]
Abstract
The transition dipole moment (TDM) orientation in the emission layer (EML) of organic light-emitting diodes (OLEDs) have attracted increasing attention from many researchers. But the study point at the molecular orientation in the hole transport layer (HTL) and electron transport layer (ETL) was not reported widely. In this paper, the molecular orientation of HTLs and ETLs were controlled by the deposition rate. The angle-dependent PL spectra and the variable angle spectroscopic ellipsometry (VASE) were used for evaluating the molecular orientation of B3PYMPM and TAPC, respectively. We found that fast deposition rate can boost preferentially vertical molecular orientation in both molecules and facilitate the hole and electron mobility, which was tested by the current density-voltage and capacitance-voltage curves of HODs and EODs. Moreover, the HTLs and ETLs were employed in OLED devices to verify the influence of molecular orientation on charge carrier mobility, which determined the performance of OLEDs significantly.
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8
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Sukegawa Y, Sato K, Fujiwara W, Katagiri H, Yokoyama D. Effect of the conformer distribution on the properties of amorphous organic semiconductor films for organic light-emitting diodes. Phys Chem Chem Phys 2021; 23:14242-14251. [PMID: 34159982 DOI: 10.1039/d1cp00892g] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
With the remarkable improvement in the electrical and optical properties of organic light-emitting diodes (OLEDs) in recent years, the details of the higher-order structure of vacuum-deposited amorphous organic films and its formation mechanism need to be understood. In particular, to clarify the effect of the higher-order structure on the film properties, it is necessary to analyze the molecular aggregation states in the vacuum-deposited amorphous films. Toward their deep understanding, the higher-order structure and film properties have often been discussed with relation to the surface diffusion and structural relaxation of the molecules immediately after deposition on the film surface. However, the effect of the variety of conformers, which is specific to amorphous organic materials, on the thermal and electrical properties of the films has not been deeply discussed. In this study, we focused on three structural isomers of OLED materials and discuss the effect of the conformer distribution on the molecular aggregation states and thermal and electrical properties of the vacuum-deposited films. From their comparison, we found that the properties of the film composed of a relatively small number of stable conformers are superior to those of the other two films composed of relatively large numbers of stable conformers. This superiority originates from formation of aggregates of the same conformer, which become the starting points for crystallization when the film is heated. Our detailed comparison and discussion focusing on the variety of conformers will lead to a deeper understanding of the molecular aggregation states and physical properties of amorphous organic films.
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Affiliation(s)
- Yoshihito Sukegawa
- Department of Organic Materials Science, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan.
| | - Kaito Sato
- Department of Polymer Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan
| | - Wataru Fujiwara
- Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan
| | - Hiroshi Katagiri
- Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan
| | - Daisuke Yokoyama
- Department of Organic Materials Science, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan. and Department of Polymer Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan and Research Center for Organic Electronics (ROEL), Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan
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9
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Bagchi K, Ediger MD. Controlling Structure and Properties of Vapor-Deposited Glasses of Organic Semiconductors: Recent Advances and Challenges. J Phys Chem Lett 2020; 11:6935-6945. [PMID: 32787194 DOI: 10.1021/acs.jpclett.0c01682] [Citation(s) in RCA: 23] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
The past decade has seen great progress in manipulating the structure of vapor-deposited glasses of organic semiconductors. Upon varying the substrate temperature during deposition, glasses with a wide range of density and molecular orientation can be prepared from a given molecule. We review recent studies that show the structure of vapor-deposited glasses can be tuned to significantly improve the external quantum efficiency and lifetime of organic light-emitting diodes (OLEDs). We highlight the ability of molecular simulations to reproduce experimentally observed structures, setting the stage for in silico design of vapor-deposited glasses in the coming decade. Finally, we identify research opportunities for improving the properties of organic semiconductors by controlling the structure of vapor-deposited glasses.
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Affiliation(s)
- Kushal Bagchi
- Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States
| | - M D Ediger
- Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States
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10
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Wang Z, Du T, Anoop Krishnan NM, Smedskjaer MM, Bauchy M. On the equivalence of vapor-deposited and melt-quenched glasses. J Chem Phys 2020; 152:164504. [DOI: 10.1063/5.0006590] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022] Open
Affiliation(s)
- Zhe Wang
- Physics of AmoRphous and Inorganic Solids Laboratory (PARISlab), Department of Civil and Environmental Engineering, University of California, Los Angeles, California 90095, USA
| | - Tao Du
- Physics of AmoRphous and Inorganic Solids Laboratory (PARISlab), Department of Civil and Environmental Engineering, University of California, Los Angeles, California 90095, USA
- School of Civil Engineering, Harbin Institute of Technology, 150090 Harbin, China
| | - N. M. Anoop Krishnan
- Department of Civil Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
- Department of Materials Science and Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
| | - Morten M. Smedskjaer
- Department of Chemistry and Bioscience, Aalborg University, 9220 Aalborg, Denmark
| | - Mathieu Bauchy
- Physics of AmoRphous and Inorganic Solids Laboratory (PARISlab), Department of Civil and Environmental Engineering, University of California, Los Angeles, California 90095, USA
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11
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Boehm BJ, Nguyen HTL, Huang DM. The interplay of interfaces, supramolecular assembly, and electronics in organic semiconductors. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2019; 31:423001. [PMID: 31212263 DOI: 10.1088/1361-648x/ab2ac2] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
Organic semiconductors, which include a diverse range of carbon-based small molecules and polymers with interesting optoelectronic properties, offer many advantages over conventional inorganic semiconductors such as silicon and are growing in importance in electronic applications. Although these materials are now the basis of a lucrative industry in electronic displays, many promising applications such as photovoltaics remain largely untapped. One major impediment to more rapid development and widespread adoption of organic semiconductor technologies is that device performance is not easily predicted from the chemical structure of the constituent molecules. Fundamentally, this is because organic semiconductor molecules, unlike inorganic materials, interact by weak non-covalent forces, resulting in significant structural disorder that can strongly impact electronic properties. Nevertheless, directional forces between generally anisotropic organic-semiconductor molecules, combined with translational symmetry breaking at interfaces, can be exploited to control supramolecular order and consequent electronic properties in these materials. This review surveys recent advances in understanding of supramolecular assembly at organic-semiconductor interfaces and its impact on device properties in a number of applications, including transistors, light-emitting diodes, and photovoltaics. Recent progress and challenges in computer simulations of supramolecular assembly and orientational anisotropy at these interfaces is also addressed.
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Affiliation(s)
- Belinda J Boehm
- Department of Chemistry, School of Physical Sciences, The University of Adelaide, SA 5005, Australia
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12
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Vapor deposition of a nonmesogen prepares highly structured organic glasses. Proc Natl Acad Sci U S A 2019; 116:21421-21426. [PMID: 31527259 DOI: 10.1073/pnas.1908445116] [Citation(s) in RCA: 24] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
We show that glasses with aligned smectic liquid crystal-like order can be produced by physical vapor deposition of a molecule without any equilibrium liquid crystal phases. Smectic-like order in vapor-deposited films was characterized by wide-angle X-ray scattering. A surface equilibration mechanism predicts the highly smectic-like vapor-deposited structure to be a result of significant vertical anchoring at the surface of the equilibrium liquid, and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy orientation analysis confirms this prediction. Understanding of the mechanism enables informed engineering of different levels of smectic order in vapor-deposited glasses to suit various applications. The preparation of a glass with orientational and translational order from a nonliquid crystal opens up an exciting paradigm for accessing extreme anisotropy in glassy solids.
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13
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Abstract
The term "organic solids" encompasses both crystals and glasses. Organic crystals are commonly grown for purification and structure determination and are being extensively explored for applications in organic electronics including field effect transistors. The ability to control the packing of one molecule relative to its neighbors is of critical importance for most uses of organic crystals. Often, anisotropic packing is also highly desirable as it enhances charge transport and optimizes light absorption/emission. When compared to crystals, the local packing in organic glasses is highly disordered and often isotropic. Glasses, however, offer two key advantages with respect to crystals. First, glasses typically lack grain boundaries and thus exhibit better macroscopic homogeneity. Second, glass composition can often be varied over a wide range while maintaining homogeneity. Besides electronic materials, many modern plastics used in a wide range of technologies are organic glasses, and the glassy state is being increasingly utilized to deliver pharmaceuticals because of higher bioavailability. In this article, we introduce vapor-deposited organic glasses as hybrid materials that combine some of the useful features of crystals and traditional liquid-cooled glasses. Physical vapor deposition produces glasses by directly condensing molecules from the gas phase onto a temperature-controlled substrate and allows film thickness to be controlled with nanometer precision. Just as liquid-cooled glasses, vapor-deposited glasses have smooth surfaces and lack grain boundaries. These attributes are critical for applications such as organic light emitting diodes (OLEDs), in which vapor-deposited glasses of organic semiconductors form the active layers. In common with crystals, vapor-deposited glasses can exhibit anisotropic packing, and the extent of anisotropy can be comparable to that of the typical organic crystal. For vapor-deposited glasses, in contrast to crystals, anisotropic packing can generally be controlled as a continuous variable. Deposition conditions can be chosen to produce glasses with significant molecular orientation (molecules "standing up" or "lying down" relative to the substrate), and π-stacking can be directed along different directions relative to the substrate. Over the last five years, we have gained a fundamental understanding of the mechanism that controls the anisotropy of vapor-deposited glasses and learned how to control many aspects of anisotropic packing. Two key elements that enable such control are the high mobility present at the surface of an organic glass and the tendency of the surface to promote anisotropic packing of molecules. In contrast to traditional epitaxial growth, for vapor-deposited glasses, the free surface (not the substrate) acts as a template that controls the structure of a growing film. The structure of any given layer is decoupled from those beneath it, thereby providing considerable freedom in producing layered glassy structures.
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Affiliation(s)
- M. D. Ediger
- Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States
| | - Juan de Pablo
- Institute of Molecular Engineering, University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois 60637, United States
| | - Lian Yu
- Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States
- School of Pharmacy, University of Wisconsin-Madison, 777 Highland Avenue, Madison, Wisconsin 53705, United States
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14
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Morgenstern T, Schmid M, Hofmann A, Bierling M, Jäger L, Brütting W. Correlating Optical and Electrical Dipole Moments To Pinpoint Phosphorescent Dye Alignment in Organic Light-Emitting Diodes. ACS APPLIED MATERIALS & INTERFACES 2018; 10:31541-31551. [PMID: 30136569 DOI: 10.1021/acsami.8b08963] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Understanding the morphology of organic materials within optoelectronic thin film devices is of crucial importance for the development of state-of-the-art organic light emitting diodes (OLEDs). In this context, the preferential alignment of organometallic Ir complexes has been in the focus of research to benefit from the improved light-outcoupling efficiencies. Although the emissive dipole orientation has been identified from an optical point of view and molecular dynamic simulations give first insights into film morphologies, new experimental techniques are necessary to pinpoint the exact alignment of phosphorescent dye molecules. In this work, optical characterization of luminescent thin films was combined with electrical measurements on bilayer devices to elucidate the orientation distribution of both, electrical and optical dipole moments of phosphorescent guest-host systems. The results not only confirm previous suggestions for the alignment mechanism of organometallic dyes but also disclose a direct correlation between the degree of electrical and optical dipole alignment, thus opening a roadway for achieving higher light-outcoupling efficiency in OLEDs.
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Affiliation(s)
- Thomas Morgenstern
- Institute of Physics , University of Augsburg , 86135 Augsburg , Germany
| | - Markus Schmid
- Institute of Physics , University of Augsburg , 86135 Augsburg , Germany
| | - Alexander Hofmann
- Institute of Physics , University of Augsburg , 86135 Augsburg , Germany
| | - Markus Bierling
- Institute of Physics , University of Augsburg , 86135 Augsburg , Germany
| | - Lars Jäger
- Institute of Physics , University of Augsburg , 86135 Augsburg , Germany
| | - Wolfgang Brütting
- Institute of Physics , University of Augsburg , 86135 Augsburg , Germany
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15
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Esaki Y, Komino T, Matsushima T, Adachi C. Enhanced Electrical Properties and Air Stability of Amorphous Organic Thin Films by Engineering Film Density. J Phys Chem Lett 2017; 8:5891-5897. [PMID: 29139292 DOI: 10.1021/acs.jpclett.7b02808] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
The influences of film density and molecular orientation on the carrier conduction and air stability of vacuum-deposited amorphous organic films of N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (α-NPD) were investigated. The substrate temperature (Tsub) during vacuum deposition had different effects on the film density and molecular orientation of α-NPD. Film density was a concave function of Tsub; maximum density was attained at Tsub = 270-300 K. α-NPD molecules were randomly oriented at Tsub = 342 K, and their horizontal orientation on the substrate became dominant as Tsub decreased. Hole current and air stability were clearly raised by increasing the film density by 1 to 2%; these effects were, respectively, attributed to enhanced carrier hopping between neighboring α-NPD molecules and suppressed penetration of oxygen and water. These results imply that increasing film density is more effective to enhance the electrical performance of organic thin-film devices with α-NPD films than control of molecular orientation.
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Affiliation(s)
- Yu Esaki
- Center for Organic Photonics and Electronics Research (OPERA), Kyushu University , 744 Motooka, Nishi, Fukuoka 819-0395, Japan
- Japan Science and Technology Agency (JST) , ERATO, Adachi Molecular Exciton Engineering Project, 744 Motooka, Nishi, Fukuoka 819-0395, Japan
| | - Takeshi Komino
- Center for Organic Photonics and Electronics Research (OPERA), Kyushu University , 744 Motooka, Nishi, Fukuoka 819-0395, Japan
- Japan Science and Technology Agency (JST) , ERATO, Adachi Molecular Exciton Engineering Project, 744 Motooka, Nishi, Fukuoka 819-0395, Japan
- Education Center for Global Leaders in Molecular System for Devices, Kyushu University , 744 Motooka, Nishi, Fukuoka 819-0395, Japan
| | - Toshinori Matsushima
- Center for Organic Photonics and Electronics Research (OPERA), Kyushu University , 744 Motooka, Nishi, Fukuoka 819-0395, Japan
- Japan Science and Technology Agency (JST) , ERATO, Adachi Molecular Exciton Engineering Project, 744 Motooka, Nishi, Fukuoka 819-0395, Japan
- International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University , 744 Motooka, Nishi, Fukuoka 819-0395, Japan
| | - Chihaya Adachi
- Center for Organic Photonics and Electronics Research (OPERA), Kyushu University , 744 Motooka, Nishi, Fukuoka 819-0395, Japan
- Japan Science and Technology Agency (JST) , ERATO, Adachi Molecular Exciton Engineering Project, 744 Motooka, Nishi, Fukuoka 819-0395, Japan
- Education Center for Global Leaders in Molecular System for Devices, Kyushu University , 744 Motooka, Nishi, Fukuoka 819-0395, Japan
- International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University , 744 Motooka, Nishi, Fukuoka 819-0395, Japan
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16
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Berthier L, Charbonneau P, Flenner E, Zamponi F. Origin of Ultrastability in Vapor-Deposited Glasses. PHYSICAL REVIEW LETTERS 2017; 119:188002. [PMID: 29219597 DOI: 10.1103/physrevlett.119.188002] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/08/2017] [Indexed: 06/07/2023]
Abstract
Glass films created by vapor-depositing molecules onto a substrate can exhibit properties similar to those of ordinary glasses aged for thousands of years. It is believed that enhanced surface mobility is the mechanism that allows vapor deposition to create such exceptional glasses, but it is unclear how this effect is related to the final state of the film. Here we use molecular dynamics simulations to model vapor deposition and an efficient Monte Carlo algorithm to determine the deposition rate needed to create ultrastable glassy films. We obtain a scaling relation that quantitatively captures the efficiency gain of vapor deposition over bulk annealing, and demonstrates that surface relaxation plays the same role in the formation of vapor-deposited glasses as bulk relaxation does in ordinary glass formation.
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Affiliation(s)
- Ludovic Berthier
- Laboratoire Charles Coulomb, UMR 5221, Université de Montpellier and CNRS, 34095 Montpellier, France
| | - Patrick Charbonneau
- Department of Chemistry, Duke University, Durham, North Carolina 27708, USA Department of Physics, Duke University, Durham, North Carolina 27708, USA
| | - Elijah Flenner
- Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, USA
| | - Francesco Zamponi
- Laboratoire de physique théorique, Ecole normale supérieure, PSL Research University, Sorbonne Universités, UPMC Univ. Paris 06, CNRS, 75005 Paris, France
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17
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Liu T, Exarhos AL, Alguire EC, Gao F, Salami-Ranjbaran E, Cheng K, Jia T, Subotnik JE, Walsh PJ, Kikkawa JM, Fakhraai Z. Birefringent Stable Glass with Predominantly Isotropic Molecular Orientation. PHYSICAL REVIEW LETTERS 2017; 119:095502. [PMID: 28949582 DOI: 10.1103/physrevlett.119.095502] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/01/2017] [Indexed: 06/07/2023]
Abstract
Birefringence in stable glasses produced by physical vapor deposition often implies molecular alignment similar to liquid crystals. As such, it remains unclear whether these glasses share the same energy landscape as liquid-quenched glasses that have been aged for millions of years. Here, we produce stable glasses of 9-(3,5-di(naphthalen-1-yl)phenyl)anthracene molecules that retain three-dimensional shapes and do not preferentially align in a specific direction. Using a combination of angle- and polarization-dependent photoluminescence and ellipsometry experiments, we show that these stable glasses possess a predominantly isotropic molecular orientation while being optically birefringent. The intrinsic birefringence strongly correlates with increased density, showing that molecular ordering is not required to produce stable glasses or optical birefringence, and provides important insights into the process of stable glass formation via surface-mediated equilibration. To our knowledge, such novel amorphous packing has never been reported in the past.
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Affiliation(s)
- Tianyi Liu
- Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
| | - Annemarie L Exarhos
- Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
| | - Ethan C Alguire
- Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
| | - Feng Gao
- Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
| | | | - Kevin Cheng
- Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
| | - Tiezheng Jia
- Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
| | - Joseph E Subotnik
- Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
| | - Patrick J Walsh
- Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
| | - James M Kikkawa
- Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
| | - Zahra Fakhraai
- Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
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18
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Zhang K, Li Y, Huang Q, Wang B, Zheng X, Ren Y, Yang W. Ultrastable Amorphous Sb 2Se 3 Film. J Phys Chem B 2017; 121:8188-8194. [PMID: 28792754 DOI: 10.1021/acs.jpcb.7b03408] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
Increasing the thermostability of amorphous materials has been a long journey to improve their properties. The metastable nature of chalcogenide glasses limits their practical applications as an amorphous semiconductor in photovoltaic performance. Here, we report the formation and physical properties of ultrastable amorphous Sb2Se3 with an enhanced thermal stability compared to ordinary amorphous Sb2Se3 (ΔTx= 17 K). By in situ high temperature-high energy synchrotron X-ray diffraction, the difference in structure relaxation between ordinary and ultrastable amorphous Sb2Se3 was manifested by local structure evolution. Ultrastable amorphous Sb2Se3 showed the smallest surface roughness and highest refractive index, the mechanism behind was further discussed in terms of fast molecular mobility and molecular orientation during vapor deposition. Formation of ultrastable amorphous Sb2Se3 demonstrated a promising avenue to obtain novel functional amorphous semiconductor with modulated structure and property.
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Affiliation(s)
- Kai Zhang
- Center for High Pressure Science and Technology Advanced Research (HPSTAR) , Shanghai 201203, China
| | - Yang Li
- Wuhan National Laboratory for Optoelectronics and School of Optical and Electronic Information, Huazhong University of Science and Technology (HUST) , Wuhan 430074, China
| | - Quan Huang
- Center for High Pressure Science and Technology Advanced Research (HPSTAR) , Shanghai 201203, China
| | - Bihan Wang
- Center for High Pressure Science and Technology Advanced Research (HPSTAR) , Shanghai 201203, China
| | - Xuerong Zheng
- School of Materials Science and Engineering, Key Laboratory of Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin University , Tianjin 300072, China
| | - Yang Ren
- X-Ray Science Division, Advanced Photon Source, Argonne National Laboratory , Argonne, Illinois 60439, United States
| | - Wenge Yang
- Center for High Pressure Science and Technology Advanced Research (HPSTAR) , Shanghai 201203, China.,High Pressure Synergetic Consortium (HPSynC), Geophysical Laboratory, Carnegie Institution of Washington , Argonne, Illinois 60439, United States
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19
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Walters DM, Antony L, de Pablo JJ, Ediger MD. Influence of Molecular Shape on the Thermal Stability and Molecular Orientation of Vapor-Deposited Organic Semiconductors. J Phys Chem Lett 2017; 8:3380-3386. [PMID: 28677392 DOI: 10.1021/acs.jpclett.7b01097] [Citation(s) in RCA: 32] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
High thermal stability and anisotropic molecular orientation enhance the performance of vapor-deposited organic semiconductors, but controlling these properties is a challenge in amorphous materials. To understand the influence of molecular shape on these properties, vapor-deposited glasses of three disk-shaped molecules were prepared. For all three systems, enhanced thermal stability is observed for glasses prepared over a wide range of substrate temperatures and anisotropic molecular orientation is observed at lower substrate temperatures. For two of the disk-shaped molecules, atomistic simulations of thin films were also performed and anisotropic molecular orientation was observed at the equilibrium liquid surface. We find that the structure and thermal stability of these vapor-deposited glasses results from high surface mobility and partial equilibration toward the structure of the equilibrium liquid surface during the deposition process. For the three molecules studied, molecular shape is a dominant factor in determining the anisotropy of vapor-deposited glasses.
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Affiliation(s)
- Diane M Walters
- Department of Chemistry, University of Wisconsin-Madison , Madison, Wisconsin 53706, United States
| | - Lucas Antony
- Institute for Molecular Engineering, University of Chicago , Chicago, Illinois 60637, United States
| | - Juan J de Pablo
- Institute for Molecular Engineering, University of Chicago , Chicago, Illinois 60637, United States
| | - M D Ediger
- Department of Chemistry, University of Wisconsin-Madison , Madison, Wisconsin 53706, United States
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20
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Glor EC, Ferrier RC, Li C, Composto RJ, Fakhraai Z. Out-of-plane orientation alignment and reorientation dynamics of gold nanorods in polymer nanocomposite films. SOFT MATTER 2017; 13:2207-2215. [PMID: 28243639 DOI: 10.1039/c6sm02403c] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
In this work, we develop a novel, in situ characterization method to measure the orientation order parameter and investigate the reorientation and reshaping dynamics of polymer grafted gold nanorods (AuNRs) in polymer nanocomposite (PNC) thin films. The long aspect-ratio of AuNRs results in two well-defined plasmon resonance modes, allowing the optical properties of the PNC to be tuned over a wide spectral range. The alignment of the AuNRs in a particular direction can also be used to further tune these optical properties. We utilize variable angle spectroscopic ellipsometry as a unique technique to measure the optical properties of PNC films containing AuNRs at various angles of incidence, and use effective index of refraction analysis of the PNC to relate the birefringence in the film due to changes of the plasmon coupling to the orientation order parameter of AuNRs. Polymer thin films (ca. 70 nm) of either polystyrene (PS) or poly(methyl methacrylate) (PMMA) containing PS grafted AuNRs are probed with ellipsometry, and the resulting extinction coefficient spectra compare favorably with more traditional analytical techniques, electron microscopy (EM) and optical absorbance (vis-NIR) spectroscopy. Furthermore, variable angle spectroscopic ellipsometry measures optical birefringence, which allows us to determine the in- and out-of plane order of the AuNRs, a property that is not easily accessible using other measurement techniques. Additionally, this technique is applied in situ to demonstrate that AuNRs undergo a rapid (ca. 1-5 hours) reorientation before undergoing a slower (ca. 24 hours) rod to sphere shape transition. The reorientation behavior is different depending on the polymer matrix used. In the athermal case (i.e. PS matrix), the AuNRs reorient isotropically, while in PMMA the AuNRs do not become isotropic, which we hypothesize is due to PMMA preferentially wetting the silica substrate, leaving less vertical space for the AuNRs to reorient.
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Affiliation(s)
- Ethan C Glor
- Department of Chemistry, University of Pennsylvania, Philadelphia, PA, USA.
| | - Robert C Ferrier
- Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, PA, USA
| | - Chen Li
- Department of Chemistry, University of Pennsylvania, Philadelphia, PA, USA.
| | - Russell J Composto
- Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA, USA.
| | - Zahra Fakhraai
- Department of Chemistry, University of Pennsylvania, Philadelphia, PA, USA.
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21
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Laventure A, Gujral A, Lebel O, Pellerin C, Ediger MD. Influence of Hydrogen Bonding on the Kinetic Stability of Vapor-Deposited Glasses of Triazine Derivatives. J Phys Chem B 2017; 121:2350-2358. [DOI: 10.1021/acs.jpcb.6b12676] [Citation(s) in RCA: 24] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Audrey Laventure
- Département
de chimie, Université de Montréal, C.P. 6128, Succ. Centre-Ville, Montréal, Québec H3C 3J7, Canada
| | - Ankit Gujral
- Department
of Chemistry, University of Wisconsin−Madison, Madison, Wisconsin 53706, United States
| | - Olivier Lebel
- Department
of Chemistry and Chemical Engineering, Royal Military College of Canada, Kingston, Ontario K7K 7B4, Canada
| | - Christian Pellerin
- Département
de chimie, Université de Montréal, C.P. 6128, Succ. Centre-Ville, Montréal, Québec H3C 3J7, Canada
| | - M. D. Ediger
- Department
of Chemistry, University of Wisconsin−Madison, Madison, Wisconsin 53706, United States
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22
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Gómez J, Gujral A, Huang C, Bishop C, Yu L, Ediger MD. Nematic-like stable glasses without equilibrium liquid crystal phases. J Chem Phys 2017; 146:054503. [DOI: 10.1063/1.4974829] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022] Open
Affiliation(s)
- Jaritza Gómez
- Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
| | - Ankit Gujral
- Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
| | - Chengbin Huang
- School of Pharmacy, University of Wisconsin-Madison, 777 Highland Avenue, Madison, Wisconsin 53705-2222, USA
| | - Camille Bishop
- Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
| | - Lian Yu
- School of Pharmacy, University of Wisconsin-Madison, 777 Highland Avenue, Madison, Wisconsin 53705-2222, USA
| | - M. D. Ediger
- Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
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