47 research outputs found
Ionic-electronic interaction in optoelectronic and sensing devices
En este trabajo se estudian las interacciones entre cargas electrónicas e iónicas y sus aplicaciones en sensores y dispositivos optoelectrónicos. El mecanismo de funcionamiento de los dispositivos optoelectrónicos actualmente más comunes, como los transistores de efecto de campo de capa fina (TFTs), los diodos emisores de luz (LEDs) y las células solares se basa solamente en procesos electrónicos, lo que significa que los efectos de cargas iónicas están ausentes, son irrelevantes o incluso perjudiciales para el propósito de dichos dispositivos. Sin embargo, muchas aplicaciones se benefician de la presencia de iones en la estructura de un dispositivo. Un ejemplo típico en el campo de investigación para la búsqueda de fuentes de luz eficientes y baratas son las células electroquémicas emisoras de luz (en inglés light-emitting electrochemical cells, con acrónimo LECs). Estos dispositivos están basados en una única capa activa de un material electroluminiscente iónico intercalada entre dos electrodos, uno de los cuales es transparente. En los p-LECs (basados en polémeros electroluminiscentes) los iones en la capa activa se añaden en forma de sales, mientras que en los iTMC-LECs (basados en compuesto iónicos de metales de transición, ionic transition metal complex, iTMCs) forman parte del complejo electroluminiscente. A menudo, al complejo electroluminiscente se añaden también líquidos iónicos con el fin de mejorar las prestaciones del dispositivo. Cuando se aplica un voltaje Vbias a los contactos externos del dispositivo, los iones en la capa activa migran hacia los electrodos, donde generan un campo interfacial que disminuye labarrera de inyección de cargas electrónicas desde los mismos electrodos. Como consecuencia, electronesy huecos son inyectados en los niveles LUMO y HOMO del polímero o del complejo electroluminiscente (HOMO es el acrónimo de highest occupied molecular level, e indica el nivel de energía molecular ocupado más elevado, el análogo para los semiconductores orgánicos de la banda de valencia de los semiconductores inorgánicos; asimismo, LUMO, acrónimo de lowest unoccupied molecular orbitalindica el nivel energético molecular libre más bajo, el análogo para conductores orgánicos de la banda de conducción de los semiconductores inorgánicos). Las cargas inyectadas difunden en el volumen de la capa activa, donde se recombinan radiativamente emitiendo fotones (luz). La inyección de carga asistida por iones se autoregula, permitiendo la fabricación de un dispositivo emisor de luz sencillo y potencialmente barato, cuyo comportamiento es prácticamente independiente de la función de trabajo de los electrodos. El equivalente "no iónico" de los LECs son los más conocidos, y ya comercializados, diodos orgánicos emisores de luz (organic light-emitting diodes, OLEDs). Los OLEDs actualmente más avanzados son dispositivos multicapa cuya fabricación requiere procesos de deposición en múltiples pasos por medio de técnicas de vacío. Estas técnicas son lentas y costosas y limitan la viabilidad comercial de los OLEDs como alternativa económica para el mercado de la iluminación. Las técnicas de deposición por disolución en múltiples pasos son difíciles de poner en práctica, porque el disolvente necesario para depositar una capa puede dañar las capas subyacentes o ser incompatible con ella, generando problemas de uniformidad durante la deposición. En el caso de los OLEDs, el enfoque multicapa es necesarios, a fin de integrar en la estructura del dispositivo materiales para la inyección y el transporte de electrones y huecos y facilitar su transporte y su confinamiento hasta el interior de la capa electroluminiscente. Además, a diferencia de los LECs, los OLEDs a menudo requieren una encapsulación rigurosa (es decir, el contacto con el aire y la humedad tiene que ser eliminado o reducido al extremo), no sólo para proteger el material orgánico, sino también porque metales muy reactivos como Ba o Ca, con función de trabajo baja, son necesarios para asegurar una eficiente inyección de electrones desde el cátodo.
Como mencionado anteriormente, estos requisitos no son indispensables en el caso de los LECs. Por lo tanto, en relación a la facilidad de fabricación y a la necesidad de encapsulación, el diferente principio de funcionamiento de los LECs los favorece respecto a los OLEDs, disminuyendo el coste final del dispositivo. Sin embargo, es importante puntualizar que mientras los LECs son un campo de investigación casi exclusivamente académico, los OLEDs representan una tecnología ya madura que se está usando en aplicaciones comerciales, como por ejemplo pantallas para televisores y teléfonos móviles. Por otro lado, la presencia de iones móviles en los LECs hace que éstos puedan potencialmente competir en un futuro en aplicaciones prácticas en las cuales el coste final sea un factor predominante a la hora de elegir entre las tecnologías disponibles.
En muchos casos, la interacción entre los electrones y los iones puede mejorar las prestaciones de dispositivos ya existentes, como en el caso de los transistores de puerta electrolítica (electrolyte-gated transistors, EGTs). Estos dispositivos son los homólogos de los transistores de película delgada de estado sólido (thin- film transistors, TFTs). Los EGTs y los TFTs comparten el mismo principio de funcionamiento, siendo ambos transistores de efecto de campo, pero en los EGTs el aislante de la puerta se substituye por un electrolito líquido o polimérico en contacto directo con el canal semiconductor. Por medio de la aplicación de un voltaje de la puerta, los iones en el electrolitose acumulan en la interfaz con el semiconductor, formando lo que se conoce como una doble capa eléctrica (electrical double layer, EDL). Dado que la capacitancia de dicha EDL es mucho más alta que la de los aislantes comúnmente usados en los TFTs, y teniendo en cuenta que la corriente en un transistor es proporcional a dicha capacitancia, la corriente en los EGTs se puede modular usando voltajes de puerta extremadamente bajos (por lo general, inferior a 3V). Además, la capacitancia del EDL y las prestaciones de un EGT son en principio independientes del grosor del electrolito, simplificando la fabricación del transistor. Lo que es aún más importante, el hecho de que el electrolito esté en contacto directo con el semiconductor implica que éste puede ser modificado o funcionalizado, o que la misma arquitectura del transistor se puede variar, para que el dispositivo funcione come sensor de iones o moléculas en ambiente acuoso o fisiológico.
Más recientemente, otra clase de dispositivos cuyo comportamiento está profundamente influenciado por las interacciones entre iones y electrones está recibiendo mucha atención por parte de la comunidad científica. Estos dispositivos son las células solares basadas en perovskitas híbridas orgánicas-inorgánicas (perovskite solar cells, PSCs), materiales con la estructura cristalina de tipo perovskita ycompuestos por cationes orgánicos intercalados en una estructura inorgánica. La perovskita más comúnmente utilizada es la de ioduro de plomo y metilammonio (CH3NH3PbI3 o MAPbI3, donde MA indica el catión orgánico metilamonio). La popularidad de las células solaresbasadas en estos materiales se debe a su extraordinaria eficiencia (que hoy en día supera el 20%), unida a la simplicidad y flexibilidad de los métodos de preparación, que van desde la deposición por técnicas de vacío al procesado por disolución. Por otro lado, pese a sus excepcionales eficiencias, las PSCs son dispositivos peculiares que, a diferencia de los dispositivos fotovoltaicos de estado sólido u orgánicos, a menudo muestran un cierto grado de histéresis en las medidas de densidad de corriente–voltaje (J–V), dependiendo de la velocidad y dirección del barrido. También son frecuentes fenómenos transitorios en la escala de tiempo de decenas o centenares de segundos, como por ejemplo un incremento gradual de la fotocorriente como consecuencia de la exposición a luz. Todo esto complica la obtención unívoca de la efectiva eficiencia de dichos dispositivos, que se determina
normalmente a partir de simple barridos J–V . Una de las hipótesis que ha sido barajada para explicar este comportamiento contempla la presencia de iones móviles en la misma perovskita. Estos iones tienen efectos relevantes en el comportamiento de las PSCs y en principio podrían ser usados para mejorar su rendimiento. En este trabajo se demuestra que la interacción entre las cargas electrónicas y las cargas iónicas se puede emplear para desarrollar dispositivos de nueva concepción basados en diseños tradicionales y, además, se muestra que la presencia de determinadas especies iónicas móviles puede influenciar de manera significativa el comportamiento de dispositivos como las células electroquímicas emisoras de luz y las células solares. En particular, se hace referencia a los siguientes casos específicos:
- La fabricación de un sensor de iones potasio K+ y de un sensor de glucosa a partir de un EGT basado en nanopartículas de ZnO como semiconductor. El sensor trabaja a bajo voltaje en un medio acuoso. El sensor de iones K+ se fabrica integrando una membrana selectiva de iones en la estructura del transistor, mientras que el sensor de glucosa se fabrica funcionalizando directamente las nanopartículas de ZnO por medio del enzima oxidasa de glucosa.
- El estudio del efecto de la adición de sales de litio con diferentes aniones en las prestaciones y el tiempo de vida de iTMC-LECs. Se muestra que el tiempo de encendido y el tiempo de vida de los LECs depende fuertemente del anión de la sal de litio. En particular, un tiempo de vida de casi 2000h se obtiene cuanto litio tetrafluoroborato es usado como aditivo.
- El estudio de la electroluminiscencia en diodos basados en la perovskita CH3NH3PbI3. Por medio de la polarización del material, los iones presentes en la perovskita se desplazan y facilitan la inyección de electrones en dispositivos con alta barriera de inyección. Esto conlleva un aumento de la electroluminiscencia y a la disminución de su voltaje de encendido, análogamente a lo que pasa para los LECs. Además, se demuestra la recuperación de la eficiencia de una célula solar degradada debido a la polarización externa, como consecuencia de la mejora en la extracción de cargas.The working mechanism of today’s most common solid-state optoelectronic
devices such as field effect thin-film transistors (TFTs), light-emitting diodes
(LEDs) and solar cells is based solely on electronic processes, meaning that only
electronic charges play a role, while the effects related to ionic charges are either
absent, irrelevant or even detrimental for their purposes. On the other hand,
many applications do actually benefit from the presence of ions in the structure
of a device. One typical example in the research field for inexpensive and efficient
lighting sources are light-emitting electrochemical cells (LECs). These are light
emitting devices composed by a single ionic electroluminescent layer sandwiched
between two electrodes. The ions in the active layer are either added during the
fabrication process (LECs based on electroluminescent polymers blended with
ionic liquids) or are part of the electroluminescent compound (LECs based on
ionic transition metal complexes (iTMCs), even if also in this case a small amount
of ionic liquid or other salts is often added to enhance the LECs performances.
The deposition of the active layer can be performed by simple solution processing
(i.e. spin coating, dip coating, meniscus coating). When a voltage bias is
applied at the external contacts, mobile ions in the active layer migrate towards
the electrodes and favor the injection of holes and electrons from the electrodes
themselves. When the injection barriers are overcome and if the bias is large
enough (higher than the band gap of the active material divided by the elementary
charge), electrons and hole are injected into the LUMO and the HOMO of
the electroluminescent material, respectively (HOMO is the acronym for highest
occupied molecular orbital, and is the analogue for organic semiconductors
of the top of the valence band of inorganic semiconductors; similarly, LUMO is
the acronym for lowest unoccupied molecular orbital, the analogue for organic
semiconductors of the bottom of conduction band of inorganic semiconductors).
The charge carriers are then driven by diffusion into the bulk of the active layer,
where they recombine radiatively. The mechanism by which the charge carrier
injection and transport take place has been object of an intense debate
(electrochemical model vs. electrodynamical model vs. unified model), nonetheless the
main principle is that the ionic-assisted charge injection allows the fabrication
of simple and potentially inexpensive light-emitting devices whose performances
are only weakly affected by the work function of the chosen electrodes, since the
charge injection is self-regulated by the ionic mechanism.
The “non-ionic” counterpart of LECs are the widely studied and already commercialized
organic light-emitting diodes (OLEDs). State-of-the-art OLEDs are
multi-layer devices whose manufacturing requires multi-step vacuum deposition
techniques, which are still too expensive to make OLEDs a viable alternative in
the mainstream lighting market. In general, multi-layer solution processing is
difficult because the solvent needed to deposit a layer has to be carefully chosen
to avoid damaging of the underlying stack of layers. This step, moreover, can
introduce wetting issues which complicate the uniform deposition of the desired
material. In the case of OLEDs, the multilayer approach is used in order to integrate
the structure of the device with electron- and hole-injection and transport
layers to facilitate the optimal displacement and confinement of charge carriers
into the bulk of the electroluminescent material. Moreover, unlike LECs, OLEDs
need rigorous encapsulation not only to protect the organic materials from air
or moisture, but also because to ensure an efficient electron injection low work
function, reactive metals such as Ba or Ca can be used as the cathode. These
constraints can be circumvented in LECs, at least partially.
It is important to point out that LECs and OLEDs show great differences in
their performances such as efficiency, lifetime, and turn-on time. In general, while
LECs are still an ongoing research topic, OLEDs are a mature technology that
is already being used in commercial applications such as displays.
In many cases the interplay between ions and electrons can boost the performances of already existing devices, as in the case of electrolyte-gated transistors
(EGTs), that have their counterpart in traditional solid-state TFTs. As will be
shown more in detail in the following sections, EGTs share the same working
principle of TFTs, meaning that they are fundamentally (organic) field-effect
transistors, (O)FETs, in which the gate insulator is replaced by a suitable
electrolyte in direct contact with the semiconductor channel. Under the application
of a small gate bias (typically below 3V), the ions in the electrolyte accumulate
at the interface of the semiconductor, forming a thin electrical double layer
(EDL). As the area capacitance of the EDL is much higher compared to that of
common insulator, and considering that the current modulation in an (O)FET
is proportional to that same capacitance, EGTs can be operated at a very low
voltage. Moreover, unlike in classical (O)FETs, the performances of EGTs are
in principle independent of the thickness of the electrolyte. This translates in a
much simpler fabrication because processes such as drop casting or spin coating
can be employed for the deposition of the electrolyte. Also, the organic semiconductor
itself can be functionalized or, alternatively, the architecture of the
transistor can be modified to make it suitable as a sensor for biologically relevant
ions or analytes.
More recently, another class of devices whose behavior is influenced by the
interaction between ions and electrons has gained popularity among the research
community. These are the solar cells based on hybrid organic-inorganic perovskites.
Perovskite is the name of the crystalline structure of the photoactive
material. The hybrid character of perovskites used in photovoltaics comes from
these materials being composed by organic cations intercalated in an inorganic
framework. The most commonly used perovskite for photovoltaics is methylammonium
lead iodide (CH 3 NH 3 PbI 3 ). The popularity of the solar cells based on
this material is due to the extraordinary photovoltaic performances that they
can deliver, together with the flexibility of the preparation methods, ranging
from vacuum deposition to simple solution processes.
Despite their outstanding performances, perovskite solar cells (PSCs) are rather
peculiar devices because, unlike classical solid state or even organic solar cells,
they often present a certain degree of hysteresis in their current density–voltage
measurements (J–V ), depending on the speed and direction of the voltage scan,
and also transient phenomena in photocurrent on timescales of the order of hundreds
of seconds are often observed. This leads to uncertainties about the “true”
performances of these devices, which cannot be inferred confidently by a simple
J–V scan. While many attempts have been made to explain and mitigate such
a particular behavior, one of the hypotheses involves the presence of mobile ions
in the perovskite layer itself. These ions influence the operation of a PSC and
as such they can be used to tune or improve the extraction of photogenerated
charges.
In this work we demonstrate that the interplay between the ionic and the
electronic charges can be employed as a tool for the development of new devices
based on traditional paradigms, and show how the presence of given mobile ionic
species can influence the behavior of devices such as light-emitting electrochemical
cells and solar cells. In particular, this has been demonstrated using the following
specific cases:
- The fabrication and characterization of an EGT based on the zinc oxide
semiconductor (ZnO) that works at low voltage in an aqueous environment.
The EGT is integrated with an ion-selective membrane to serve as an ion
sensor for the selective measurement of K+ ions in an electrolyte. Moreover,
the transistor is used as a glucose sensor through the functionalization of
the ZnO with the glucose oxidase enzyme.
- The influence of lithium salts as substitutes for ionic liquids on the performances
and lifetime of iTMC LECs. The behavior of the LECs is shown to
be heavily dependent on the lithium counterion. A lifetime close to 2000h
is achieved using lithium tetrafluoroborate as the additive.
- The electroluminescence of a perovskite solar cell under bias. The effect of
the bias on a unipolar device is the onset of an ambipolar charge injection
regime and an improved electroluminescence, just like in the case of light-
emitting electrochemical cells. Moreover, the efficiency of a degraded device
can be partially recovered after biasing it for few minutes. The presence of
mobile ions in the perovskite explains the similarities between the processe
Effects of high-pressure on the structural, vibrational, and electronic properties of monazite-type PbCrO4
We have performed an experimental study of the crystal structure,
lattice-dynamics, and optical properties of PbCrO4 (the mineral crocoite) at
ambient and high pressures. In particular, the crystal structure, Raman-active
phonons, and electronic band-gap have been accurately determined.
X-ray-diffraction, Raman, and optical-absorption experiments have allowed us
also to completely characterize two pressure-induced structural phase
transitions. The first transition is isostructural, maintaining the monoclinic
symmetry of the crystal, and having important consequences in the physical
properties; among other a band-gap collapse is induced. The second one involves
an increase of the symmetry of the crystal, a volume collapse, and probably the
metallization of PbCrO4. The results are discussed in comparison with related
compounds and the effects of pressure in the electronic structure explained.
Finally, the room-temperature equation of state of the low-pressure phases is
also obtained.Comment: 32 pages, 9 figures, 3 table
APLICACIÓN DE COMPETENCIAS EN EL ÁREA DE GEOLOGÍA EN EL PROYECTO TUNING
Tuning América Latina es un trabajo conjunto que busca y construye lenguaje y sistemas de carácter transicional y transregional. En el marco del Proyecto Alfa Tuning América Latina se constituyó el grupo del área de geología (año 2006), conformado por representantes de universidades de los países de América Latina. El objetivo general es aplicar las competencias genéricas y específicas en dicha área. Ha sido concebido como un espacio de reflexión de actores comprometidos con la educación profesional, que a través de la búsqueda de consensos, avancen en el desarrollo de titulaciones fácilmente comparables y comprensibles de forma articulada en toda América Latina. La idea de llevar adelante una propuesta como la de Tuning en América Latina, surge en Europa pero planteados por latinoamericanos. La metodología empleada se fundamentó en las consultas sobre los comportamientos específicos notándose coincidencias en los procesos de formación académica en todos los países de América Latina que participaron, aplicándose un método estadístico descriptivo, considerándose el cálculo de la media y la desviación estándar de la variable obtenida de las encuestas. Se analizó y discutió el panorama del área temática en los distintos países, se analizaron las competencias genéricas elaboradas en etapas anteriores del proyecto, se definieron las competencias específicas y los métodos de consulta para su evaluación. En los resultados se resalta la importancia de obtener un espectro amplio, tanto de países e instituciones con el fin de consolidar dicho proyecto
Controlling the mode of operation of organic transistors through side-chain engineering
Electrolyte-gated organic transistors offer low bias operation facilitated by direct contact of the transistor channel with an electrolyte. Their operation mode is generally defined by the dimensionality of charge transport, where a field-effect transistor allows for electrostatic charge accumulation at the electrolyte/semiconductor interface, whereas an organic electrochemical transistor (OECT) facilitates penetration of ions into the bulk of the channel, considered a slow process, leading to volumetric doping and electronic transport. Conducting polymer OECTs allow for fast switching and high currents through incorporation of excess, hygroscopic ionic phases, but operate in depletion mode. Here, we show that the use of glycolated side chains on a thiophene backbone can result in accumulation mode OECTs with high currents, transconductance, and sharp subthreshold switching, while maintaining fast switching speeds. Compared with alkylated analogs of the same backbone, the triethylene glycol side chains shift the mode of operation of aqueous electrolyte-gated transistors from interfacial to bulk doping/transport and show complete and reversible electrochromism and high volumetric capacitance at low operating biases. We propose that the glycol side chains facilitate hydration and ion penetration, without compromising electronic mobility, and suggest that this synthetic approach can be used to guide the design of organic mixed conductors
Efficient vacuum deposited p-i-n and n-i-p perovskite solar cells employing doped charge transport layers
Methylammonium lead halide perovskites have emerged as high performance photovoltaic materials. Most of these solar cells are prepared via solution-processing and record efficiencies (>20%) have been obtained employing perovskites with mixed halides and organic cations on (mesoscopic) metal oxides. Here, we demonstrate fully vacuum deposited planar perovskite solar cells by depositing methylammonium lead iodide in between intrinsic and doped organic charge transport molecules. Two configurations, one inverted with respect to the other, p-i-n and n-i-p, are prepared and optimized leading to planar solar cells without hysteresis and very high efficiencies, 16.5% and 20%, respectively. It is the first time that a direct comparison between these two opposite device configurations has been reported. These fully vacuum deposited solar cells, employing doped organic charge transport layers, validate for the first time vacuum based processing as a real alternative for perovskite solar cell preparation
Pressure-Driven Symmetry-Preserving Phase Transitions in Co(IO3)(2)
[EN] High-pressure synchrotron X-ray diffraction studies of cobalt iodate, Co(IO3)(2), reveal a counterintuitive pressure-induced expansion along certain crystallographic directions. High-pressure Raman and infrared spectroscopy, combined with density-functional theory calculations, reveal that with increasing pressure, it becomes energetically favorable for certain I-O bonds to increase in length over the full range of pressure studied up to 28 GPa. This phenomenon is driven by the high-pressure behavior of iodate ion lone electron pairs. Two pressure-induced isosymmetric monoclinic-monoclinic phase transitions are observed at around 3.0 and 9.0 GPa, which are characterized by increasing oxygen coordination of the iodine atoms and the probable formation of pressure-induced metavalent bonds. Pressure-volume equations of state are presented, as well as a detailed discussion of the pressure dependences of the observed Raman- and infrared-active modes, which clarifies previous inconsistencies in the literature.This work was supported by the Generalitat Valenciana under Project PROMETEO 2018/123-EFIMAT and by the Spanish Ministerio de Ciencia, Universidades, e Investigacion under Projects PID2019-106383GB-41/42/43, as well as through MALTA Consolider Team research network (RED2018102612-T). A.M. and P.R.-H. acknowledge computing time provided by Red Espan~ola de Supercomputacion (RES) and the MALTA Consolider Team cluster. D.E. acknowledges the resources and technical assistance provided by the Informatics Service of Universitat de Valencia through the Tirant III cluster. A.L. and D.E. would like to thank the Generalitat Valenciana for the Ph.D. Fellowship no. GRISOLIAP/2019/025. R.T. acknowledges funding from the Spanish Ministerio de Economia y Competitividad (MINECO) via the Juan de la Cierva Formacion fellowship (FJC2018-036185-I). C.P. is thankful for the financial support of the Spanish Mineco Project no. FIS2017-83295-P. E.B would like to thank the University of Valencia for his "Attraccio de Talent" postdoctoral contract (UV-INV_POSTDOC19-1026935). The authors thank Sandrine Beauquis from Symme, Universite Savoie Mont Blanc (France), for her technical assistance concerning the SEM and ADX analyses. PXRD experiments were performed at the MSPD beamline of ALBA Synchrotron (experiment no. 2019083663). IR experiments were performed at the MIRAS beamline of ALBA Synchrotron (experiment no. 2020024118).Liang, A.; Popescu, C.; Manjón, F.; Turnbull, R.; Bandiello, E.; Rodriguez-Hernandez, P.; Muñoz, A.... (2021). Pressure-Driven Symmetry-Preserving Phase Transitions in Co(IO3)(2). The Journal of Physical Chemistry C. 125(31):17448-17461. https://doi.org/10.1021/acs.jpcc.1c0465917448174611253
Molecular Design of Semiconducting Polymers for High-Performance Organic Electrochemical Transistors.
The organic electrochemical transistor (OECT), capable of transducing small ionic fluxes into electronic signals in an aqueous environment, is an ideal device to utilize in bioelectronic applications. Currently, most OECTs are fabricated with commercially available conducting poly(3,4-ethylenedioxythiophene) (PEDOT)-based suspensions and are therefore operated in depletion mode. Here, we present a series of semiconducting polymers designed to elucidate important structure-property guidelines required for accumulation mode OECT operation. We discuss key aspects relating to OECT performance such as ion and hole transport, electrochromic properties, operational voltage, and stability. The demonstration of our molecular design strategy is the fabrication of accumulation mode OECTs that clearly outperform state-of-the-art PEDOT-based devices, and show stability under aqueous operation without the need for formulation additives and cross-linkers
Layered topological semimetal GaGeTe: New polytype with non-centrosymmetric structure
[EN]
GaGeTe is a layered van der Waals material composed of germanene and GaTe sublayers that has been recently predicted to be a basic Z2 topological semimetal. To date, only one polytype of GaGeTe is known with trigonal centrosymmetric structure (a phase, space group R-3m, No. 166). Here we show that asgrown samples of GaGeTe show traces of at least another polytype with hexagonal noncentrosymmetric structure (f3 phase, space group P63mc, No. 186). Moreover, we suggest that another bulk hexagonal polytype (g phase, space group P-3m1, No. 164) could also be found near room conditions. Both a and f3 polytypes have been identified and characterized by means of X-ray diffraction and Raman scattering measurements with the support of ab initio calculations. We provide the vibrational properties of both polytypes and show that the Raman spectrum reported for GaGeTe almost forty years ago and attributed to the a phase, was, in fact, that of the secondary f3 phase. Additionally, we show that a Fermi resonance occurs in a-GaGeTe under non-resonant excitation conditions, but not under resonant excitation conditions. Theoretical calculations show that bulk f3-GaGeTe is a non-centrosymmetric weak topological semimetal with even smaller lattice thermal conductivity than centrosymmetric bulk aGaGeTe. In perspective, our work paves the way for the control and engineering of GaGeTe polytypes to design and implement complex van der Waals heterostructures formed by a combination of centrosymmetric and non-centrosymmetric layers of up to three different polytypes in a single material, suitable for a number of fundamental studies and technological applications.This publication is part of the project MALTA Consolider Team network (RED2018-102612-T) , financed by MINECO/AEI/10.13039/501100003329; by I ? D ? i projects PID2019-106383 GB -41/42/43 financed by MCIN/AEI/10.13039/501100011033; and by project PROMETEO/2018/123 (EFIMAT) financed by Generalitat Valenciana. E.B. would like to thank the Universitat Politecnica de Valencia for his postdoctoral contract (Ref. PAID -10-21) . AHR was supported by the U.S. Department of Energy (DOE) , Office of Science, Basic Energy Sciences under award DE-SC0021375. We also acknowledge the computational resources awarded by XSEDE, a project supported by National Science Foundation grant number ACI-1053575. The authors also acknowledge the support from the Texas Advances Computer Center (with the Stampede2 and Bridges supercom- puters) . E.L.d.S would like to acknowledge the Network of Extreme Conditions Laboratories (NECL) , financed by FCT and co -financed by NORTE 2020, through the program Portugal 2020 and FEDER; the High Performance Computing Chair-a R & D infrastructure (based at the University of ? Evora; PI: M. Avillez) ; and for the computational support provided by the HPC center OBLIVION -U. ? Evora to perform the lattice thermal conductivity calculations. A.L. and D.E. would like to thank the Generalitat Valenciana for the Ph.D. Fellowship no. GRISOLIAP/2019/025.Gallego-Parra, S.; Bandiello, E.; Liang, A.; Da Silva, EL.; Rodriguez-Hernandez, P.; Muñoz, A.; Radescu, S.... (2022). Layered topological semimetal GaGeTe: New polytype with non-centrosymmetric structure. Materials Today Advances. 16:1-16. https://doi.org/10.1016/j.mtadv.2022.1003091161