| 1. |
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- 2019
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Tidskriftsartikel (refereegranskat)
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| 2. |
- Mink, Janos, et al.
(författare)
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Vibrational properties of -KSiH3 and -RbSiH3 : a combined Raman and inelastic neutron scattering study
- 2017
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Ingår i: Journal of Raman Spectroscopy. - : Wiley. - 0377-0486 .- 1097-4555. ; 48:2, s. 284-291
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Tidskriftsartikel (refereegranskat)abstract
- The hydrogen storage materials ASiH(3) (A=K and Rb) represent complex metal hydrides built from metal cations and pyramidal SiH3- ions. At room temperature, SiH3- moieties are randomly oriented because of dynamical disorder (-modifications). At temperatures below 200K, ASiH(3) exist as ordered low-temperature () modifications. The vibrational properties of -ASiH(3) were characterized by a combination of Raman spectroscopy and inelastic neutron scattering. Internal modes of SiH3- are observed in the spectral range 1800-1900cm(-1) (stretching modes) and 890-1000cm(-1) (bending modes). External modes are observed below 500cm(-1). Specifically, SiH3- librations are between 300-450cm(-1) and 270-400cm(-1) for A=K and Rb, respectively, SiH3- translations are between 95 and 160cm(-1), K+ translations are in the range 60-100cm(-1) and Rb+ translations in the range 50-70cm(-1). The red-shift of libration modes for A=Rb is associated with a 15-30% reduction of the libration force constants of SiH3- ions in -RbSiH3. This correlates with a lower temperature for the - order-disorder phase transition (278 vs 298K). Libration modes become significantly anharmonic with increasing temperature but are maintained up to at least 200K. The vibrational properties of ASiH(3) compare well to those of alkali metal borohydrides ABH(4) (A=Na-Cs).
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| 3. |
- Nedumkandathil, Reji, et al.
(författare)
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Investigation of the Order–Disorder Rotator Phase Transition in KSiH3 and RbSiH3
- 2017
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 121:9, s. 5241-5252
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Tidskriftsartikel (refereegranskat)abstract
- The β–α (order–disorder) transition in the silanides ASiH3 (A = K, Rb) was investigated by multiple techniques, including neutron powder diffraction (NPD, on the corresponding deuterides), Raman spectroscopy, heat capacity (Cp), solid-state 2H NMR spectroscopy, and quasi-elastic neutron scattering (QENS). The crystal structure of α-ASiH3 corresponds to a NaCl-type arrangement of alkali metal ions and randomly oriented, pyramidal, SiH3– moieties. At temperatures below 200 K ASiH3 exist as hydrogen-ordered (β) forms. Upon heating the transition occurs at 279(3) and 300(3) K for RbSiH3 and KSiH3, respectively. The transition is accompanied by a large molar volume increase of about 14%. The Cp(T) behavior is characteristic of a rotator phase transition by increasing anomalously above 120 K and displaying a discontinuous drop at the transition temperature. Pronounced anharmonicity above 200 K, mirroring the breakdown of constraints on SiH3– rotation, is also seen in the evolution of atomic displacement parameters and the broadening and eventual disappearance of libration modes in the Raman spectra. In α-ASiH3, the SiH3– anions undergo rotational diffusion with average relaxation times of 0.2–0.3 ps between successive H jumps. The first-order reconstructive phase transition is characterized by a large hysteresis (20–40 K). 2H NMR revealed that the α-form can coexist, presumably as 2–4 nm (sub-Bragg) sized domains, with the β-phase below the phase transition temperatures established from Cp measurements. The reorientational mobility of H atoms in undercooled α-phase is reduced, with relaxation times on the order of picoseconds. The occurrence of rotator phases α-ASiH3 near room temperature and the presence of dynamical disorder even in the low-temperature β-phases imply that SiH3– ions are only weakly coordinated in an environment of A+ cations. The orientational flexibility of SiH3– can be attributed to the simultaneous presence of a lone pair and (weakly) hydridic hydrogen ligands, leading to an ambidentate coordination behavior toward metal cations.
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| 4. |
- Kranak, Verina F., et al.
(författare)
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Structural and Vibrational Properties of Silyl (SiH3-) Anions in KSiH3 and RbSiH3 : New Insight into Si-H Interactions
- 2015
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 54:5, s. 2300-2309
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Tidskriftsartikel (refereegranskat)abstract
- The alkali metal silyl hydrides ASiH(3) (A = K, Rb) and their deuteride analogues were prepared from the Zintl phases ASi. The crystal structures of ASiH(3) consist of metal cations and pyramidal SiH3 ions. At room temperature SiH3 moieties are randomly oriented (alpha modifications). At temperatures below 200 K ASiH(3) exist as ordered low-temperature (beta) modifications. Structural and vibrational properties of SiH3- in ASiH(3) were characterized by a combination of neutron total scattering experiments, infrared and Raman spectroscopy, as well as density functional theory calculations. In disordered alpha-ASiH(3) SiH3 ions relate closely to freely rotating moieties with C(3)v symmetry (Si-H bond length = 1.52 angstrom; HSiH angle 92.2 degrees). Observed stretches and bends are at 1909/1903 cm(-1) (nu(1), A(1)), 1883/1872 cm(-1) (nu(3), E), 988/986 cm(-1) (nu(4), E), and 897/894 cm(-1) (nu(2), A(1)) for A = K/Rb. In ordered beta-ASiH(3) silyl anions are slightly distorted with respect to their ideal C-3v symmetry. Compared to a-ASiH(3) the molar volume is by about 15% smaller and the SiH stretching force constant is reduced by 4%. These peculiarities are attributed to reorientational dynamics of SiH3 anions in a-ASiH(3). SiH stretching force constants for SiH3 moieties in various environments fall in a range from 1.9 to 2.05 N cm(-1). These values are considerably smaller compared to silane, SiH4 (2.77 N cm(-1)). The reason for the drastic reduction of bond strength in SiH3- remains to be explored.
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| 5. |
- Lin, Chih-Yuan, 1987-, et al.
(författare)
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A Comparative Analysis of Emulated and Real IEC-104 Spontaneous Traffic in Power System Networks
- 2021
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Ingår i: Cyber-Physical Security for Critical Infrastructures Protection. - Cham : Springer International Publishing. - 9783030697808 - 9783030697815 ; , s. 207-223
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Konferensbidrag (refereegranskat)abstract
- Supervisory and Data Acquisition (SCADA) systems control and monitor modern power networks. As attacks targeting SCADA systems are increasing, significant research is conducted to defend SCADA networks including variations of anomaly detection. Due to the sensitivity of real data, many defence mechanisms have been tested only in small testbeds or emulated traffic that were designed with assumptions on how SCADA systems behave. This work provides a timing characterization of IEC-104 spontaneous traffic and compares the results from emulated traffic and real traffic to verify if the network characteristics appearing in testbeds and emulated traffic coincide with real traffic. Among three verified characteristics, two of them appear in the real dataset but in a less regular way, and one does not appear in the collected real data. The insights from these observations are discussed in terms of presumed differences between emulated and real traffic and how those differences are generated.
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| 6. |
- Lin, Chih-Yuan, 1987-
(författare)
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A timing approach to network-based anomaly detection for SCADA systems
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Supervisory Control and Data Acquisition (SCADA) systems control and monitor critical infrastructure in society, such as electricity transmission and distribution systems. Modern SCADA systems are increasingly adopting open architectures, protocols, and standards and being connected to the Internet to enable remote control. A boost in sophisticated attacks against SCADA systems makes SCADA security a pressing issue. An Intrusion Detection System (IDS) is a security countermeasure that monitors a network and tracks unauthenticated activities inside the network. Most commercial IDSs used in general IT systems are signature-based, by which an IDS compares the system behaviors with known attack patterns. Unfortunately, recent attacks against SCADA systems exploit zero-day vulnerabilities in SCADA devices which are undetectable by signature-based IDSs.This thesis aims to enhance SCADA system monitoring by anomaly detection that models normal behaviors and finds deviations from the model. With anomaly detection, zero-day attacks are possible to detect. We focus on modeling the timing attributes of SCADA traffic for two reasons: (1) the timing regularity fits the automation nature of SCADA systems, and (2) the timing information (i.e., arrival time) of a packet is captured and sent by a network driver where an IDS is located. Hence, it’s less prone to intentional manipulation by an attacker, compared to the payload of a packet.This thesis first categorises SCADA traffic into two groups, request-response and spontaneous traffic, and studies data collected in three different protocol formats (Modbus, Siemens S7, and IEC-60870-5-104). The request-response traffic is generated by a polling mechanism. For this type of traffic, we model the inter-arrival times for each command and response pair with a statistical approach. Results presented in this thesis show that request-response traffic exists in several SCADA traffic sets collected from systems with different sizes and settings. The proposed statistical approach for request-response traffic can detect attacks having subtle changes in timing, such as a single packet insertion and TCP prediction for two of the three SCADA protocols studied.The spontaneous traffic is generated by remote terminal units when they see significant changes in measurement values. For this type of traffic, we first use a pattern mining approach to find the timing characteristics of the data. Then, we model the suggested attributes with machine learning approaches and run it on traffic collected in a real power facility. We test our anomaly detection model with two types of attacks. One causes persistent anomalies and another only causes intermittent ones. Our anomaly detector exhibits a 100% detection rate with at most 0.5% false positive rate for the attacks with persistent anomalies. For the attacks with intermittent anomalies, we find our approach effective when (1) the anomalies last for a longer period (over 1 hour), or (2) the original traffic has relatively low volume.
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| 7. |
- Lin, Chih-Yuan, 1987-
(författare)
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Network-based Anomaly Detection for SCADA Systems : Traffic Generation and Modeling
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Supervisory Control and Data Acquisition (SCADA) systems control and monitor critical infrastructure in society, such as electricity transmission and distribution systems. Modern SCADA systems are increasingly adopting open standards and being connected to the Internet to enable remote control. A boost in sophisticated attacks against SCADA systems makes SCADA security a pressing issue. An Intrusion Detection System (IDS) is a security countermeasure that monitors a network and tracks unauthenticated activities inside the network. Most commercial IDSs used in general IT systems are signature-based, by which an IDS compares the system behaviors with known attack patterns. Unfortunately, recent attacks against SCADA systems exploit zero-day vulnerabilities which are undetectable by signature-based IDSs. This thesis aims to enhance SCADA system monitoring by network-based anomaly detection that models normal behaviors and finds deviations from the model. With network-based anomaly detection, zero-day attacks are possible to detect. There are two main challenges for network-based anomaly detection. The first challenge is the potentially large number of false positives coming from benign traffic that just deviates from the trained model due to the noises. To address this challenge, this thesis proposes several traffic modeling approaches based on statistics and machine learning techniques for the regular communication patterns in SCADA traffic. The second challenge is the lack of open datasets to evaluate the proposed approaches. Consequently, this thesis proposes a traffic generation framework. For traffic modeling, this thesis first categorises SCADA traffic into two groups, request-response and non-requested traffic, and studies data collected in a diverse set of protocol for-mats (Modbus, Siemens S7, S7+, MMS, IEC-60870-5-104). The request-response traffic is generated by a polling mechanism. For this type of traffic, we model the inter-arrival times for each request and response pair with a statistical approach. Results presented in this thesis show that request-response traffic exists in several SCADA traffic sets collected from systems with different sizes and settings. The proposed statistical approach for request-response traffic can detect attacks having subtle changes in timing. The non-requested traffic is generated by remote terminal units at predefined times or when they see significant changes in measurement values. For this type of traffic, we first use a pattern mining approach to find the timing characteristics of the data. Then, we model the suggested attributes with machine learning approaches. We test our anomaly detection model with two types of attacks. One causes persistent anomalies and another only causes intermittent ones. Our anomaly detector exhibits a 100% detection rate with at most 0.5% false positive rate for the attacks with persistent anomalies. For the attacks with intermittent anomalies, we find our approach effective when anomalous patterns last for a longer period (over 30 minutes). For traffic generation, this thesis conducts a comparative analysis between network traces collected from testbeds and a real power utility. The analysis shows that the testbed traffic may be prone to overly regular patterns. This is considered to be the result of lack of plausible human interactions within the testbed. Therefore, this thesis proposes a traffic generation framework built upon a virtual testbed. The framework provides programmable BOTs to mimic human activities such as commands from the operators and attacks.
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| 8. |
- Lin, Chih-Yuan, 1987-, et al.
(författare)
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RICSel21 Data Collection : Attacks in a Virtual Power Network
- 2021
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Ingår i: 2021 IEEE International Conference on Communications, Control, and Computing Technologies for Smart Grids (SmartGridComm). - : Institute of Electrical and Electronics Engineers (IEEE). - 9781665430449 - 9781665415026 ; , s. 201-206
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Konferensbidrag (refereegranskat)abstract
- Attacks against Supervisory Control and Data Acquisition (SCADA) systems operating critical infrastructures have increased since the appearance of Stuxnet. To defend critical infrastructures, security researchers need realistic datasets to evaluate and benchmark their defense mechanisms such as Anomaly Detection Systems (ADS). However, real-world data collected from critical infrastructures are too sensitive to share openly. Therefore, testbed datasets have become a viable option to balance the requirement of openness and realism. This study provides a data generation framework based on a virtual testbed with a commercial SCADA system and presents an openly available dataset called RICSel21, with packets in IEC-60870-5-104 protocol streams. The dataset is the result of performing 12 attacks, identifying the impact of attacks on a power management system and recording the logs of the seven successful attacks.
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| 9. |
- Lin, Chih-Yuan, 1987-, et al.
(författare)
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Timing-Based Anomaly Detection in SCADA Networks
- 2018
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Ingår i: Critical Information Infrastructures Security. - Cham : Springer. - 9783319998428 - 9783319998435 ; , s. 48-59
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Konferensbidrag (refereegranskat)abstract
- Supervisory Control and Data Acquisition (SCADA) systems that operate our critical infrastructures are subject to increased cyber attacks. Due to the use of request-response communication in polling, SCADA traffic exhibits stable and predictable communication patterns. This paper provides a timing-based anomaly detection system that uses the statistical attributes of the communication patterns. This system is validated with three datasets, one generated from real devices and two from emulated networks, and is shown to have a False Positive Rate (FPR) under 1.4%. The tests are performed in the context of three different attack scenarios, which involve valid messages so they cannot be detected by whitelisting mechanisms. The detection accuracy and timing performance are adequate for all the attack scenarios in request-response communications. With other interaction patterns (i.e. spontaneous communications), we found instead that 2 out of 3 attacks are detected.
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| 10. |
- Lin, Chih-Yuan, 1987-, et al.
(författare)
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Timing Patterns and Correlations in Spontaneous SCADA Traffic for Anomaly Detection
- 2019
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Ingår i: PROCEEDINGS OF THE 22ND INTERNATIONAL SYMPOSIUM ON RESEARCH IN ATTACKS, INTRUSIONS AND DEFENSES. - : USENIX - The Advanced Computing Systems Association. - 9781939133076 ; , s. 73-88
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Konferensbidrag (refereegranskat)abstract
- Supervisory Control and Data Acquisition (SCADA) systems operate critical infrastructures in our modern society despite their vulnerability to attacks and misuse. There are several anomaly detection systems based on the cycles of polling mechanisms used in SCADA systems, but the feasibility of anomaly detection systems based on non-polling traffic, so called spontaneous events, is not well-studied. This paper presents a novel approach to modeling the timing characteristics of spontaneous events in an IEC-60870-5-104 network and exploits the model for anomaly detection. The system is tested with a dataset from a real power utility with injected timing effects from two attack scenarios. One attack causes timing anomalies due to persistent malfunctioning in the field devices, and the other generates intermittent anomalies caused by malware on the field devices, which is considered as stealthy. The detection accuracy and timing performance are promising for all the experiments with persistent anomalies. With intermittent anomalies, we found that our approach is effective for anomalies in low-volume traffic or attacks lasting over 1 hour.
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| 11. |
- Lin, Chih-Yuan, 1987-, et al.
(författare)
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Understanding IEC-60870-5-104 Traffic Patterns in SCADA Networks
- 2018
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Ingår i: Proceedings of the 4th ACM Workshop on Cyber-Physical System Security. - NY, USA : ACM. - 9781450357555 ; , s. 51-60
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Konferensbidrag (refereegranskat)abstract
- The IEC-60870-5-104 (IEC-104) protocol is commonly used in Supervisory Control and Data Acquisition (SCADA) networks to operate critical infrastructures, such as power stations. As the importance of SCADA security is growing, characterization and modeling of SCADA traffic for developing defense mechanisms based on the regularity of the polling mechanism used in SCADA systems has been studied, whereas the characterization of traffic caused by non-polling mechanisms, such as spontaneous events, has not been well-studied. This paper provides a first look at how the traffic flowing between SCADA components changes over time. It proposes a method built upon Probabilistic Suffix Tree (PST) to discover the underlying timing patterns of spontaneous events. In 11 out of 14 tested data sequences, we see evidence of existence of underlying patterns. Next, the prediction capability of the approach, useful for devising anomaly detection mechanisms, is studied. While some data patterns enable an 80% prediction possibility, more work is needed to tune the method for higher accuracy.
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| 12. |
- Lin, Yuan-Chih, 1987
(författare)
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Dynamic local structural symmetries and luminescence properties of the yellow emitting phosphor Ce3+-doped Y3Al5O12
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis centers on investigations of the local structure and vibrational properties of the oxide garnet Y3Al5O12 (YAG), which when substituted with a few percent of the activator ion Ce3+ to replace Y3+ (Y3−xCexAl5O12, YAG:Ce3+) is one of the most important phosphors for solid state lighting. The study builds on a comprehensive analysis of the nature of the phonons and localized vibrational modes in YAG:Ce3+ and how these depend on the Ce3+ concentration and temperature and how they affect key optical properties, such as the intensity and frequency of the emitted light. The investigations have been carried out by using a combination of Raman, infrared, luminescence, and neutron spectroscopies, together with mode-selective vibrational excitation, and are further supported by computer modeling. The results show that the static and dynamic structure of YAG:Ce3+ are dependent on both Ce3+ concentration and temperature. The substitution of the heavier Ce3+ ions compared to Y3+ is found to lower the vibrational frequencies of most of the phonons and localized vibrational modes around the Ce3+ ions, implying that they become more populated at a given temperature. As the temperature increases, vibrational modes of higher and higher frequency are activated, some of which induce significant dynamical tetragonal distortions around the Ce3+ ions. These distortions are shown to lead to a red-shift of the frequency of the emitted light. In addition, some of the high-frequency phonons are shown to be notably related to non-radiative relaxation of the excited-state electrons of the Ce3+ ions via electron-phonon coupling, which decreases the emission intensity when these phonons are activated. The reduction in emission intensity at elevated temperature is however a complicated process as it is found to relate also to thermal ionization of the excited-state electrons into the conduction band of the host crystal, which may be followed by charge trapping by defects.
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| 13. |
- Lin, Yuan-Chih, 1987, et al.
(författare)
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Inorganic Phosphor Materials for Lighting
- 2016
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Ingår i: Topics in Current Chemistry. - : Springer Science and Business Media LLC. - 0340-1022 .- 2365-0869 .- 2364-8961. ; 374:2, s. 374-421
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Forskningsöversikt (refereegranskat)abstract
- This chapter addresses the development of inorganic phosphor materials capable of converting the near UV or blue radiation emitted by a light emitting diode to visible radiation that can be suitably combined to yield white light. These materials are at the core of the new generation of solid-state lighting devices that are emerging as a crucial clean and energy saving technology. The chapter introduces the problem of white light generation using inorganic phosphors and the structure-property relationships in the broad class of phosphor materials, normally containing lanthanide or transition metal ions as dopants. Radiative and non-radiative relaxation mechanisms are briefly described. Phosphors emitting light of different colors (yellow, blue, green, and red) are described and reviewed, classifying them in different chemical families of the host (silicates, phosphates, aluminates, borates, and non-oxide hosts). This research field has grown rapidly and is still growing, but the discovery of new phosphor materials with optimized properties (in terms of emission efficiency, chemical and thermal stability, color, purity, and cost of fabrication) would still be of the utmost importance.
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| 14. |
- Lin, Yuan-Chih, 1987
(författare)
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Understanding the interactions between vibrational modes and excited state relaxation in garnet structured phosphors
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis concerns investigations of the local structural environments and vibrational dynamics of the three garnet type oxide phosphors Ce3+-doped Y3Al5O12 (YAG:Ce3+), Ca3Sc2Si3O12 (CSS:Ce3+), and Sr3Y2Ge3O12 (SYG:Ce3+), which show promising optical properties as luminescent materials used in solid state white lighting technologies. The study focuses on a comprehensive analysis of the nature of the long-range vibrations (phonons) in terms of local atomic and molecular motions of the garnet structure, as well as their dependence on the nature of the garnet chemical composition, Ce3+ concentration and temperature. The aim is to understand how these materials properties aect key optical properties, such as the intensity and wavelength (color) of the emitted light. The investigations have been conducted by using a combination of Raman, infrared, luminescence, and neutron spectroscopies, together with mode-selective vibrational excitation experiments, and are further supported by theoretical and semi-empirical analyses and computer modeling based on density functional theory. The results show that increasing the Ce3+ concentration and/or temperature cause(s) a red-shifting eect on the emission color due to an increased crystal eld acting on the Ce3+ ions in YAG:Ce3+. This is primarily attributed to the thermal excitation of certain high-frequency phonon modes that induce dynamical tetragonal distortions of the local CeO8 moieties. A reversal (blue-)shift of the emission color observed at higher temperatures is, however, the result of counteracting thermal lattice expansion which turns the local coordination of CeO8 into a more cubic symmetry. Specically, it is found that the upward-shift of the frequencies of certain vibrational modes in YAG:Ce3+ through decreasing the Ce3+ concentration or cosubstitution of smaller and/or lighter atoms on the Y sites increases the thermal stability of the emission intensity. This higher thermal stability of the emission intensity is attributed to a less activation of modes that give rise to nonradiative relaxation of electrons in the excited states via electron{phonon coupling and/or energy migration processes. For SYG:Ce3+, the emission intensity is found to decrease strongly with increasing temperature, as a result of thermal ionization by promoting the electrons of Ce3+ ions into the conduction band of the host, followed by charge trapping at defects. CSS:Ce3+ exhibits excellent thermal stability up to very high temperatures, 860 K.
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| 15. |
- Lin, Yuan-Chih, 1987, et al.
(författare)
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Understanding the Interactions between Vibrational Modes and Excited State Relaxation in Y₃₋ₓCeₓAl₅O₁₂: Design Principles for Phosphors Based on 5d-4f Transitions
- 2018
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 1520-5002 .- 0897-4756. ; 30:6, s. 1865-1877
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Tidskriftsartikel (refereegranskat)abstract
- The oxide garnet Y 3 Al 5 O 12 (YAG), when a few percent of the activator ions Ce 3+ substitutes for Y 3+ , is a luminescent material widely used in phosphor-converted white lighting. However, fundamental questions surrounding the defect chemistry and luminescent per formance of this material remain, especially in regard to the nature and role of vibrational dynamics. Here, we provide a complete phonon assignment of YAG and establish the general spectral trends upon variation of the Ce 3+ dopant concentration and temperature, which are shown to correlate with the macroscopic luminescence properties of Y 3-x Ce x Al 5 O 12 . Increasing the Ce 3+ concentration and/or temperature leads to a red-shift of the emitted light, as a result of increased crystal-field splitting due to a larger tetragonal distortion of the CeO 8 moieties. Decreasing the Ce 3+ concentration or cosubstitution of smaller and/or lighter atoms on the Y sites creates the potential to suppress thermal quenching of luminescence because the frequencies of phonon modes important for nonradiative relaxation mechanisms are upward-shifted and hence less readily activated. It follows that design principles for finding new Ce 3+ -doped oxide phosphors emitting at longer wavelengths require tetragonally distorted environments around the CeO 8 moieties and a sufficiently rigid host structure and/or low activator-ion concentration to avoid thermal quenching of luminescence.
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| 16. |
- Lin, Yuan-Chih, 1987, et al.
(författare)
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Unraveling the impact of different thermal quenching routes on the luminescence efficiency of the Y3Al5O12:Ce3+ phosphor for white light emitting diodes
- 2020
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Ingår i: Journal of Materials Chemistry C. - : Royal Society of Chemistry (RSC). - 2050-7534 .- 2050-7526. ; 8:40, s. 14015-14027
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Tidskriftsartikel (refereegranskat)abstract
- Cerium doped yttrium aluminium garnet, Y3-zCezAl5O12, is the prototype material for solid-state white lighting and it still is an important white LED phosphor. However, fundamental understanding of the thermal quenching of luminescence, which leads to a pronounced reduction of the emission intensity under high-power light-emitting diode operation, remains to be obtained. Here we show, through a multitechnique approach based on photoluminescence, thermoluminescence and mode-selective vibrational excitation experiments that thermal quenching of luminescence in Y3-zCezAl5O12 is caused by a combined effect of thermal ionization, thermally activated concentration quenching, and thermally activated 5d → 4f crossover relaxation via electron-phonon coupling, and establish the general trends upon variation of the Ce3+ concentration and temperature. Thermal quenching below 600 K is primarily the result of concentration quenching and crossover relaxation, which can be suppressed by keeping the Ce3+ dopant concentration far below 0.7 mol%, whereas for temperatures above 600 K thermal ionization is the predominating quenching process. This new insight into the interplay between different thermal quenching processes provides design principles for optimizing the light emittance and colour stability of new phosphor materials used in white lighting devices characterized by certain operating temperatures. This journal is
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| 17. |
- Lin, Yuan-Chih, 1987, et al.
(författare)
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Unraveling the Mechanisms of Thermal Quenching of Luminescence in Ce3+-Doped Garnet Phosphors †
- 2019
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 1520-5002 .- 0897-4756. ; 31:11, s. 3851-3862
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Forskningsöversikt (refereegranskat)abstract
- The environmental and economic benefits of phosphor-converted white-light-emitting diodes (pc-WLEDs) have been increasingly appreciated in recent years. However, a significant challenge in this field pertains to a phenomenon known as thermal quenching, which takes place inside phosphors and leads to a pronounced reduction of the emission intensity under high-power light-emitting diode operation. The development of new, more thermally stable phosphors depends on a better understanding of the mechanisms underpinning thermal quenching in phosphors. Here we review the current understanding of thermal quenching mechanisms in Ce3+-doped garnet phosphors, which are widely considered one of the most important families of phosphors for application in pc-WLEDs. In particular, we highlight key structural and dynamical properties, such as the coordination environment of the Ce3+ ions, phonons and local vibrational modes, and structural and chemical defects, which are shown to correlate with phosphor performance. We also discuss the perspectives for future studies in this field in hopes of accelerating the development of new efficient phosphors featuring suppressed thermal quenching of luminescence.
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| 18. |
- Lin, Yuan-Chih, 1987, et al.
(författare)
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Vibrationally induced color shift tuning of photoluminescence in Ce³⁺-doped garnet phosphors
- 2019
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Ingår i: Journal of Materials Chemistry C. - : Royal Society of Chemistry (RSC). - 2050-7534 .- 2050-7526. ; 7:41, s. 12926-12934
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Tidskriftsartikel (refereegranskat)abstract
- A critical challenge in the field of phosphor converted white light emitting diodes (pc-WLEDs) pertains to understanding and controlling the variation of emission color with device temperature. Here we, through a combined photoluminescence (PL) and Raman spectroscopy study of the three garnet type phosphors Ce3+-doped Y3Al5O12 (YAG:Ce3+), Ca3Sc2Si3O12 (CSS:Ce3+), and Sr3Y2Ge3O12 (SYG:Ce3+), show that the color of the PL is systematically shifted upon changing the operation temperature of the phosphor. A general trend is observed that the PL exhibits a red-shift as a function of increasing temperature, until the point at which the vibrational modes of the CeO8 moieties, which induce dynamical tetragonal distortions of the CeO8 dodecahedra, are fully activated. Upon further temperature increase, the PL turns to a blue-shift because of a counteracting and predominating effect of thermal lattice expansion that progressively makes the CeO8 dodecahedra more cubal like. Since this behavior is the result of the symmetry relations intrinsic to the garnet structure, the present mechanism can be generally applicable to materials of this type. It thereby provides a route for tuning the PL of this important class of phosphor materials.
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| 19. |
- Perrichon, Adrien, 1988, et al.
(författare)
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Local Coordination Environments and Vibrational Dynamics of Protons in Hexagonal and Cubic Sc-Doped BaTiO3 Proton-Conducting Oxides
- 2020
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 124:16, s. 8643-8651
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Tidskriftsartikel (refereegranskat)abstract
- The proton local coordination environments and vibrational dynamics associated with the two order of magnitude change in proton conductivity in hydrated forms of hexagonal and cubic structured BaTi1-xScxO3Hx (0.16 < x < 0.7) were investigated using optical spectroscopy, neutron spectroscopy, and first-principles calculations. Whereas the cubic structure compositions display a single proton site, we show that protons occupy three distinct sites in compositions exhibiting the hexagonal structure. The principal site is characterized by interoctahedral hydrogen bonds, while two additional low occupancy sites are similar to those in the cubic structure, with classic intraoctahedral geometry. Furthermore, the proton hydrogen bond strength increases with decreasing scandium doping level. We infer from this that the stronger, more energetic hydrogen bonds in the hexagonal structure, resulting from proton sites with lower symmetry (lower multiplicity), are predominantly responsible for the significant reduction in macroscopic conductivity between cubic and hexagonal BaTi1-xScxO3Hx materials, rather than simply the absolute number of protons. Our findings are highly relevant to the field, clarifying the advantages of high-symmetry structures with high-multiplicity proton sites to favorable properties in ceramic proton-conducting oxides.
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| 20. |
- Sharma, Suchinder, 1982, et al.
(författare)
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Weak thermal quenching of the luminescence in the Ca3Sc2Si3O12:Ce3+ garnet phosphor
- 2018
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Ingår i: Journal of Materials Chemistry C. - : Royal Society of Chemistry (RSC). - 2050-7534 .- 2050-7526. ; 6:33, s. 8923-8933
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Tidskriftsartikel (refereegranskat)abstract
- We report results of the luminescence properties of the three garnet type phosphors Ce3+-doped Ca3Sc2Si3O12(CSSO:Ce3+), Sr3Y2Ge3O12(SYG:Ce3+) and Y3Al5O12(YAG:Ce3+), investigated using optical spectroscopy techniques and vacuum referred binding energy (VRBE) diagram analysis. By monitoring the temperature dependence of the luminescence decay time we establish an excellent, intrinsic, thermal stability of luminescence in CSSO:Ce3+, with a nearly constant decay time (≈60 ns) up to, at least, T = 860 K. In comparison, SYG:Ce3+and YAG:Ce3+exhibit a significant reduction of the luminescence decay time upon heating, starting at around T = 280 K and T = 550 K, respectively, suggesting a lower internal thermal stability of luminescence in these two garnet phosphors. These findings are supported by the energy separation between the Ce3+5d1level and the conduction band (CB) of the respective hosts, which are found at 1.36 eV (CSSO:Ce3+), 0.45 eV (SYG:Ce3+), and 1.17 eV (YAG:Ce3+), respectively, as predicted by their VRBE diagrams. The performance of CSSO:Ce3+was evaluated by applying the phosphor on a blue InGaN LED. The system shows a luminous efficacy of optical radiation of 243 lm W-1and a linear response with increasing applied voltage, suggesting it is a highly promising phosphor for future technological applications, particularly at high temperature operating environments.
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| 21. |
- Svenningsson, Leo, 1990, et al.
(författare)
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Molecular Orientation Distribution of Regenerated Cellulose Fibers Investigated with Polarized Raman Spectroscopy
- 2019
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Ingår i: Macromolecules. - : American Chemical Society (ACS). - 1520-5835 .- 0024-9297. ; 52:10, s. 3918-3924
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Tidskriftsartikel (refereegranskat)abstract
- The molecular orientation distribution of polymeric fibers influences physical properties. We present a novel method of analyzing polarized Raman experiments to determine molecular orientation, which is based on exchanging the Legendre polynomial approach with a wrapped Lorentzian function, as determined from a prescreening of X-ray scattering patterns. This method removes the need for performing right angle scattering experiments while avoiding common approximations. The molecular orientation of regenerated cellulose fibers, using the presented method, is shown to correlate well with X-ray scattering and an analogous experiment using solid-state NMR spectroscopy. Challenges of quantitatively measuring molecular anisotropy occur with semi-crystalline, partially modified, or composite materials. As such, a plethora of techniques, each with a unique chemical selectivity, is paramount for material characterization.
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