Graphene quantum dots (GQDs) and carbon dots (CDs) have been attracting research attention as carbon-based and environmentally friendly alternative of the inorganic semiconductor quantum dots. Similar to their inorganic counterpart, GQDs and CDs demonstrate promising optical properties with emission wavelength dependent on their sizes. However, their precise chemical structures are unknown and uncontrollable in most of the cases, which makes it difficult to fine-tune the properties or establish the structure-property relationship. To this end, the methods of synthetic organic chemistry provide the bottom-up approach to obtain GQDs with atomically precise structures, namely large polycyclic aromatic hydrocarbons (PAHs) that are also called nanographenes [1]. We have synthesized dibenzo[hi,st]ovalene (DBOV) as a molecular GQD with a combination of armchair and zigzag edges, which demonstrated strong red emission and stimulated emission [2]. Regioselective bromination of DBOV could be established, enabling the edge-functionalization to install various functional groups for the fine-tuning of the optical and electronic properties [3]. For example, fluoranthene imide groups were attached to DBOV, which led to red-shifted emission with larger Stokes shift [4]. The π-extension of DBOV was achieved through Pd-catalyzed alkyne benzannulation, affording circumpyrene as GQD with multiple zigzag edges [5]. On the other hand, nitrogen-doped DBOV showed acid- and metal-sensitive fluorescence, indicating its potential for sensing applications [6]. We have more recently also synthesized a wider variety of functionalized DBOVs as well as novel GQDs, providing further insights into the structure-property relationship. These results highlight the high potential of molecular GQDs for a wide range of applications in the optoelectronics and photonics.

Nonlinear optics is of paramount importance in several fields of science and technology: it is commonly used for frequency conversion, self-referencing of frequency combs, sensing, and ultra-short pulse generation and characterization. Large efforts have been devoted in the last years to realizing electrical and all-optical modulation of the nonlinear optical response of atomically thin materials, which are easy to integrate on photonic platforms and thus ideal for novel nanoscale devices [1][2]. In this talk, I will present different approaches to achieve large modulation of the harmonic generation in graphene and in transition metal dichalcogenides (TMDs). The first method is based on the electrical control of the graphene’s Fermi energy. This allows to tune the nonlinear multi-photon resonances occurring within the Dirac cone and thus to achieve a large electrical modulation of the third harmonic generation [3]. In addition, third harmonic generation in graphene can be modulated by tuning the electronic temperature in an all-optical approach [4]. The second example regards broadband all-optical modulation of the second harmonic generation. The concept is based on symmetry considerations and thus it is applicable to any material of the D_3h symmetry group and with deep sub-wavelength thickness, such as all monolayer TMDs. With this approach we demonstrated a 90° rotation of the second harmonic polarization on a time-scale limited only by the fundamental pulse duration. In addition, this ultrafast polarization switch can be immediately applied to realize all-optical second harmonic amplitude modulation [5].

      The development of industrial-scale, reliable, inexpensive production processes of graphene and related two-dimensional materials (GRMs)[1,2] is a key requirement for their widespread use in several application areas,[1-6] providing a balance between ease of fabrication and final product quality. In particular, in the energy sector, the production of GRMs in liquid phase [2,6] represents a simple and cost-effective pathway towards the development of GRMs-based energy devices, presenting huge integration flexibility compared to other production methods.

     In this presentation, I will first briefly introduce the key properties of GRMs. Then, I will present the strategy of BeDimensional in the production of GRMs by wet-jet milling [7] and the Industrial scale up. Afterward, I will provide a brief overview on some key applications of the as-produced GRMs, for anticorrosion coatings and energy conversion and storage devices. [3,8-16]

REFERENCES:

[1] F. Bonaccorso, et. al., Adv. Mater. 28, 6136-6166 (2016).

[2] F. Bonaccorso, et al., Materials Today, 15, 564-589, (2012).

[3] F. Bonaccorso, et. al., Nature Photonics 4, 611-622, (2010).

[4] E. Pomerantseva, F. Bonaccorso, et al., Science 366 (6468) eaan8285 (2019).

[5] G. Iannaccone F. Bonaccorso, et al., Nature Nanotech 13, 183, (2018).

[6] A. C. Ferrari, F. Bonaccorso, et al., Nanoscale, 7, 4598-4810 (2015).

[7] A. E. Del Rio Castillo et. al., Mater. Horiz. 5, 890 (2018).

[8] F. Bonaccorso, et. al., Science, 347, 1246501 (2015).

[9] L. Najafi et al., Advanced Energy Materials 8 (16), 1703212 (2018).

[10] E. Lamanna et al., Joule 4, 865-881 (2020).

[11] S. Bellani, et al., Chem. Soc. Rev. DOI: 10.1039/D1CS00106J (2021)

[12] M. Garakani, et al. Energy Storage Materials 34, 1-11 (2020).

[13] S. Bellani, et al. Nano Lett.  18, 7155-7164 (2018).

[14] A. E. Del Rio Castillo, et al., Chem. Mater. 30, 506-516 (2018).

[15] S. Bellani, et al. Nanoscale Horizons 4, 1077 (2019).

[16] S. Bellani, et al. Adv. Funct. Mater. 29, 1807659 (2019).

"This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement GrapheneCore3 - 881603"

Lasers based on organic active media have attracted great attention for many years, mainly because they enable wavelength tuning across the visible spectrum. The development of thin film organic lasers brought additional advantages of compactness, mechanical flexibility and low-cost production.[1] Among such devices, distributed feedback (DFB) lasers have demonstrated potential for applications in different areas, such as spectroscopy, optical communications and chemical and biological sensing. For some of these applications, materials operating in the near-infrared (NIR) region, where only a few organic lasers have been demonstrated up to date, are pursued. The principal reason for this lack of NIR emitters is the detrimental effect of fast non-radiative decay following the energy gap law, concentration quenching mechanisms and intra-gap absorption of charge-transfer and triplet states.[2] Hence, a proper understanding of such mechanisms and how to prevent them might lead to a new generation of NIR organic emitters. In this regard, nanographenes (NGs) are emerging materials which offer a great promise for light-emitting devices and integration with other technologies.[3]

In this presentation, the photophysical and laser properties of two series of NGs dispersed in thermoplastic polymer films will be described, both appertaining to the same family of [n,m]peri-acenoacenes in which a number m of [n]acenes are fused into a rhombic nanoflake (see figure). These four-zigzag edged NGs are an excellent platform for investigating the effect of size increase and zigzag edge elongation upon their photophysical and lasing properties.[4,5] Getting insights into such mechanisms provide useful information for the design of materials with emission in the NIR region. In particular, electronic wave function delocalization over the zigzag edge contributes to the red-shift improvement.[6] Following this approach, light amplification in the NIR has been demonstrated with two different NGs of the peri-acenoacene family.

Additionally, the suitability of these compounds for lasing applications has been proved with the manufacture of DFB lasers consisting of active films with top-layer resonators (1D relief gratings fabricated by holographic lithography).[7] The excellent control that provides the lithographic method over the resonator parameters and the quality of the laser architecture enable the manufacture of operational laser devices across the visible spectrum and entering into the NIR region by using the different NGs.

Combining two-dimensional (2D) materials with organic materials can be very attractive for applications that require flexibility and where size and weight are important parameters to be considered, such as in wearable, portable and mobile applications. Organic materials usually exhibit excellent optical absorption efficiency and photo- and temperature-induced conformational changes, while 2D materials often show relatively high carrier mobility, superior mechanical flexibility, and tunable electronic and optical properties. Combining both systems can stabilize the organic materials and lead to heterostructures with both high carrier mobility and high optical absorption efficiency, which is promising for solar energy conversion. In this work we investigate, by means of density-functional-theory calculations, heterostructures composed of pentacene molecules and transition metal dichalcogenides (TMD) for application in photovoltaic devices. We examine the interaction between the molecules and monolayer TMDs as well as the band alignment of the heterostructures, considering effects of the molecular coverage and dielectric screening. Our results show that the band edge positions of pentacene change significantly when going from the isolated molecule to the monolayer coverage, affecting the band alignment with a TMD monolayer.

Several works focused on studying the properties of 2D materials[1],[2] and on trying to optimize their production[3],[4], in order to employ them as active components for different applications. However, the use of pristine 2D materials is often not enough to target specific uses and therefore chemical functionalization can come into play as a versatile method to tune and bring additional properties to the nano sheets.

In this contribution, we will report on our work carried out on the functionalization and cross-linking of liquid-exfoliated graphene and 2D MoS₂ to produce tridimensional networks, using polymers as cross-linkers between the different nano-sheets. The chemical functionalization is the key to the construction of such networks: for graphene we employ the Tour reaction[5] to directly bind ad-hoc decorated aryl moieties on the basal plane that are then exploited to connect the sheets to each other through a polymerization reaction.  For 2D MoS₂, instead, bifunctional thiols are employed to both functionalize and cross-link the material in a one pot reaction. The formation of hydrogels is facilitated by the use of cross-linkers well-known for their gelation properties such poly-acrylates and poly ethylene glycols. A subsequent step of freeze-drying allows to obtain the corresponding aerogels, composite materials characterized by high surface areas available for exchange processes, like catalysis or charge-discharge.

The resulting networks are characterized by a combination of techniques to infer the structural and electronic properties. The electrochemical characterization further allows to infer the potential for use of this materials in electrocatalysis and in energy storage applications.

The efficient harvesting and subsequent storage of solar radiation is now becoming one the main challenges for the development of clean and sustainable energy sources. The ever increasing demand to reduce the usage of rare and toxic materials as well as the need to overcome current limitations in solar energy storage calls for environmentally-friendly approaches based on completely new concepts. In this context, hybrid inorganic/organic nanosystems employing metal oxides nanocrystals (MO NCs) and graphene quantum dots (GQDs) are emerging as potential candidates for both harvesting and storage of solar radiation due to the ability of MO NCs to accumulate electrons upon UV light absorption.[1]  Here, by means of chemical titration combined with optical spectroscopic tools we investigate the possibility for efficient multiple charge transfers in this type of hybrid nanosystems. Exploiting multiple charge transfer processes together with the combination of energy conversion and storage in a single set of materials will open new routes toward the development of efficient and compact light driven energy storage devices.

Ultrathin 2-D van der Waals (vdW) semiconductor materials have procured scientific and technological interest since the discovery of single layered graphene in 2004.¹ Similar to graphene, transition metal trichalcogenides, with the general chemical formula MPX₃ (M= 1st row transition metals, X = chalcogenides), possess fast electron transport and strong spin orbit coupling without the drawback of no bandgap. These vdW inorganic lamellar compounds are characterized by strong intralayer covalent bonding and weak vdW interaction between adjacent layers. Furthermore, they possess weak vdW interlayer interactions meaning that cleaving few- or single-layers from the bulk material is relatively simple using either mechanical or liquid exfoliation techniques. Furthermore, the transition metal atoms endow these materials with magnetic (either ferromagnetic or antiferromagnetic) and magneto-optical properties which can be utilized for new generation 2-D magnets and opto-spintronic devices. Yet, the fundamental understanding of these materials as well as the manipulation of their intrinsic magnetism via external stimuli remains to be an unexplored endeavor.

MPX₃ offers a large range of chemical compositions tunable via the M and X elements, consequently forming semiconductors with varying band gap energies ranging from UV to near- infrared. In this way the intrinsic magnetic properties that originate from the M atoms in the MPX₃ materials can also be tuned. Depending on the M atoms within the MPX₃ layers, they can be endowed with magnetic (either ferromagnetic or antiferromagnetic) and magneto-optical properties. In the case of FePS₃ antiferromagnetic Ising magnetic ordering is exhibited. Investigations of bulk and few layer FePS₃ have shown optical linear dichroism believed to be attributed to its zigzag antiferromagnetic ordering and anisotropic crystal structure. However, the intrinsic magnetic properties of FePS₃ and their effect on the material’s linear dichroism has yet to be explored. Furthermore, the anisotropic crystal structure of FePS₃ gives rise to inequivalent K(K’) points in the brilloiun zone. This suggests the possibility of valleytronic properties as seen in transition metal dichalcogenides (TMDs).^2–4 To date, these properties have not being shown in TMTCs such as FePS_3.    

In this talk, evidence of optical linear dichroism will be shown as a function of external magnetic field and temperature. Furthermore, the possibility of valleytronic properties of FePS₃ will be shown using circularly polarized excitation as a function of external magnetic field.  

In this work, we combine experimental and theoretical studies of the excitation transfer in heterostructures consisting of two different van der Waals materials: a monolayer transition metal dichalcogenide (TMD) and a 2D perovskite. Our results show that the band alignment inhibits the electron transfer. We present evidence for the excitation transfer and show that it is dominated by either non-radiative energy transfer or by hole transfer, depending on the alignment of exciton states.  

Due to the unprecedented flexibility offered by the total relaxation of lattice matching requirements, stacks of van der Waals semiconductors are currently the focus of intense investigations with view of applications in ultrathin optoelectronics. Such heterostructures (HSs) exhibit novel properties, absent from the constituent materials. Excitation transfer between the stacked layers can occur via a charge [1] or energy [2] transfer. Design of future devices requires a thorough understanding, which of the mechanisms dominates. This knowledge is, at present, lacking.

Here, we present a combined spectroscopic and density functional theory (DFT) studies of three samples: PEA₂PbI₄ /MoSe₂ (PEPI/MoSe₂), PEA₂PbI₄ /WS₂ (PEPI/WS₂) stacks (where PEA stands for phenylethylammonium) and BA₂PbI₄ /MoSe₂ (BAPI/MoSe₂, where BA is butylammonium). For all three HSs, the DFT calculations have shown that the obtained stacks exhibit a type II band  alignment with valence band edge (VBE) in the perovskite and conduction band edge (CBE) in  the TMD layer. However, the electron transfer between materials is hindered by the  presence of organic spacer layers in-between slabs of PbI₄ octahedra.

Low temperature photoluminescene (PL) mapping, PL excitation, and time-resolved PL measurements provided compelling evidence for the excitation transfer in all the studied HSs. However, the nature of the transfer depends on the energy alignment of the exciton states. Namely, in PEPI/WS₂, where the PEPI exciton is in resonance with the B-exciton of WS₂, a non-radiative resonant energy transfer together with hole transfer, are observed. On the other hand, in PEPI/MoSe₂ and BAPI/MoSe₂, where the B-exciton lies lower in energy, the resonant transfer is inhibited and a hole transfer dominates in the systems. It leads to an appearance of a long lived interlayer exciton, demonstrated for the first time in these HSs.

The growing demand for renewable energy and self-powered electronic devices is fueling the research on novel energy storage solutions. Some of the main limitations to the diffusion of photovoltaic energy are linked to the intermitting nature of this source, as well as to stability issues, conversion losses, and limitations related to the size of the devices. Here, we report a hybrid nanostructure based on Metal Oxide Nanocrystals (MO NCs) and MoS₂ monolayers capable of light-driven charging, combining in a unique system both the energy conversion and energy storage aspects.^[1-3] We achieve the direct storage of photons energy as extra positive and negative charges, separated by nanometric distances within the ultra-thin heterostructure. Photo-generated holes are quenched at the semiconductor's surface and injected into the MoS₂ monolayer, while the photo-excited electrons accumulate in the ITO nanocrystal (with an average of 75 per NC).  The careful design of similar heterostructures opens the path towards all-solid-state solutions for the direct storage of the solar energy.

Recent developments have shown that chemical synthesis is a powerful tool for controlling the size, shape, and composition of two-dimensional (2D) crystals with broad implications for their implementation in optoelectronic and quantum devices. However, the gas-phase reactions carried out by most researchers are not conducive to precise manipulation of the 2D crystal edge structure. Here we demonstrate a salt-assisted low-pressure chemical vapor deposition method, which enables growth of 2D WSe₂ monolayers whose edge morphology can be tuned from straight, to sawtooth, to fractal-like by adjusting the ratio of WO₃ to NaCl. We discuss the volatility of the metal precursor and its role in dictating the edge structure, defect density, and excitonic environment within the as-grown crystals. These studies provide important insight into new 2D crystal growth modes and synthetic strategies for producing crystals with unique structures and optoelectronic properties.

Bismuth-halide-based semiconductors have gained increasing attention for optoelectronics, owing to their low toxicity, high environmental stability under ambient conditions, and easy processability by a wide range of scalable methods. In particular, bismuth-halide-based materials have been proposed to replicate key features of the electronic structure of lead-halide perovskites that may give rise to defect tolerance, but without the toxicity limitations of the latter [1]. This talk will examine in detail the case of bismuth oxyiodide (BiOI; a layered compound), focussing on its carrier dynamics, defect tolerance and charge transport. Through density functional theory calculations, we show that the most common point defects in BiOI are resonant within the bands, or shallow within the bandgap [2]. To experimentally probe the defect tolerance of BiOI, we intentionally introduce defect states, and probe how these influence the electronic structure (as measured by photoemission spectroscopy) and charge-carrier lifetime (as measured by transient absorption spectroscopy) [3]. We develop an all-inorganic device structure, and devise a route to control the preferred orientation of the vapour-deposited BiOI films to achieve photovoltaics with external quantum efficiencies reaching up to 80% at 450 nm wavelength [2,4]. Further, we demonstrate the strong potential of BiOI for indoor light harvesting to power Internet of Things electronics [5]. We finish with a discussion of the key factors that will need to be addressed in order to further improve the performance of this material in optoelectronics, focussing especially on the role of carrier-phonon coupling on charge-carrier transport and dynamics.

Optical quality films of lead halide perovskite nanocrystals displaying quantum confinement effects can be achieved by in situ preparation and processing within the void space of insulating porous matrices.[1] In this talk, the main photophysical properties of embedded quantum dots will be reviewed, with emphasis on the possibilities that the absence of organic ligands offer to achieve control over the optical and charge transport properties. Evidence of efficient dot-to-dot transport,[2] fast photoactivation,[3] high photoluminescence quantum yield (>85%) and enhanced durability,[4] with respect to their bulk counterparts, will be provided for different films made of MAPbI₃, MAPbBr₃, CsPbI₃ and FAPbBr₃ quantum dots embedded in porous matrices. Also, the absence of ligands implies an excellent opportunity to analyse fundamental interactions, such electron-phonon coupling[5] and the response to the environment,[6] without the interference of organic capping layers. Overall, these results demonstrate that adequately designed networks of ligand-free perovskite quantum dots can be used as both light harvesters and photocarrier conductors, in an alternative configuration to those employed in previously developed QD optoelectronic devices.

Transition metal dichalcogenide (TMDC) is a class of material with a layered structure in bulk with an indirect bandgap, which becomes direct at the K point in momentum space if only a monolayer is present. This bandgap transition leads to a strong light-matter interaction such as enhanced photoluminescence (PL), compared to the bulk. Monolayer sheets of TMDC with large interaction area can easily be coupled to other low dimensional materials such as 0D nanocrystals, 1D nanowires, or other 2D-2D heterostructures. Halide perovskites are a class of materials with a structural formula ABX₃ where A and B are cations such as Cs, Pb, etc and X is halogen. Halide perovskites in both bulk and low dimension show excellent photon absorption and emission properties. Due to its large range of visible light absorption, halide perovskites have made their way and have significantly improved in the field of photovoltaics. A heterostructure combination of the two systems (0D-2D) can show new electronic properties. A plethora of options available in these two classes of materials in terms of band offsets and bandgaps can lead to both type I and II systems.

In this contribution, we present the large exciton energy funneling in a 0D-2D heterostructure of CsPbBr₃ (di-dodecyl, dimethyl ammonium as ligand) and MoSe₂ monolayer. MoSe₂ and CsPbBr₃ heterostructure form a type I structure with MoSe₂ being the lower bandgap material. We use steady-state and time-resolved µ-PL spectroscopy techniques to probe the photo-induced energy transfer between a set of different CsPbBr₃ NCs and MoSe₂. Spectroscopy results will be discussed in detail in the frame of exciton energy transfer and its efficiency will be compared to literature.

High performance microscale photodetectors which provide fast and efficient optical-to-electrical signal conversion are critical components for next-generation light-sensing applications. Two-dimensional (2D) metal halide hybrid perovskites are an emerging attractive 2D system that combine appealing optoelectronic properties, i.e. strong optical absorption, high carrier mobility, with high stability, easy-processing and low-cost manufacturing. However, photodetectors based on 2D metal halide hybrid perovskites usually exhibit low responsivity if compared to their 3D counterpart.

Herein, we exploit the improved photostability and suitable optoelectronic properties of high-quality 2D fluorinated-phenethylammonium lead iodide perovskite (F-PEA) single crystals to demonstrate the fabrication of lateral metal-F-PEA-metal junction photodetectors. These devices exhibit larger sensitivity and faster time response than reported to date in 2D perovskites. Finally, we discuss the use of these devices for high-resolution light-sensing applications.

Carbon electrode based perovskite solar cells (C-PSC) are promising candidates for commercialization of perovskite devices considering their low processing costs and extraordinary stability. However, due to the lack of hole selective layers, this device architecture still suffers from severe performance losses at the perovskite/carbon electrode interface. ^[1]

Recent advances in interface engineering by low dimentional 2D perovskites have proven to effectively passivate the surface of the 3D perovskite absorber.^[2] We introduce a 2D perovskite passivation layer as an electron blocking layer at the perovskite/carbon interface in hole selective layer free carbon electrode based perovskite solar cells. The successful passivation of the interface was assessed through X-ray diffraction, X-ray photoemission spectroscopy, and an advanced spectrally resolved photoluminescence (PL), revealing the formation of a high band gap 2D perovskite layer. We confirm the electron blocking characteristic of the 2D perovskite through electrochemical impedance spectroscopy and illumination intensity dependent J_SC-V_OC measurements of carbon electrode based perovskite devices implementing the 2D perovskite at the 3D perovskite/carbon interface. We show a substantial reduction in charge extraction and interfacial recombination yielding a record efficiency of 18.5% with an improved stability over 500 hours of continuous illumination.

We thus employ a 2D-perovskite as an electron blocking layer in hole selective layer free carbon electrode based perovskite solar cells with printable low temperature carbon electrode. We demonstrate its electron blocking characteristic at the perovskite/carbon interface effectively allowing for less charge recombination losses leading to highly efficient devices. 

Light-matter interaction pervades our everyday life and typically involves the exchange of energy between electromagnetic (EM) field and quantum states of matter. When the interaction strength is high enough to promote an exchange rate of energy faster than any other competing relaxation process, the overall light-matter system undergoes a drastic change in its pristine properties. This results into the formation of new hybrid states within the so called “strong coupling” (SC) regime [1].

In this view, hybrid systems composed by optical nanocavities and quantum dots (QDs) represents a key approach to acquire/induce new and distinctive physico-chemical properties with significant implications in fields ranging from cavity quantum electrodynamics and condensed matter physics to polariton chemistry [2], [3].

Here, we report on the SC interaction between surface plasmon polaritons (SPPs) and excitons in CdSe QDs, investigated by steady-state spectroscopic method and transient absorption measurements [4], [5], [6].

In particular the dispersion of the exciton–plasmon hybrid states revealed the typical signature of SC, i.e. anticrossing behavior. Concurrently, the presence of two distinctive bleaching signals appeared in the transient absorption spectra, whose relaxation dynamics underlined a decay of the hybrid states only slightly slower than the lifetime of bare CdSe nanoparticles, and much longer than the SPP damping time.

In addition to the exciton-plasmon coupling the very same hybrid platform was used to impart control over the vibrational energy landscape of semiconductor QDs. Specifically, we demonstrated phonon mode hybridization both in THz and Raman spectroscopies, thus confirming the possibility of altering the intrinsic phonon response of a nanomaterial using properly tailored optical nanoresonators [7].

Because magic-sized clusters (MSCs) are smaller than nanoparticles, they can mimic and provide insight into molecular-level processes and assume novel properties. In this talk I will highlight two recent discoveries on MSCs. The first is on inorganic isomerizations. Structural transformations are ubiquitous at all length scales, spanning from isomerization reactions of small molecules to solid-solid transformations in bulk crystals. Despite attempts to merge understanding of these disparate regimes by reducing the domain size down to nanocrystals (~2 nm), previous work found that bulk-like solid-solid transformation behavior still predominates at nanocrystal length scales. Here we show that MSCs—where, at (~1.5 nm) are smaller than nanocrystals but larger than small molecules—can exhibit a reversible isomeric transformation, and possess essential characteristics of both solid-solid transformations and molecular isomerization reactions. The diffusionless reconfiguration of the inorganic core is evidenced by our reconstruction of the atomic pair distribution function (PDF) from total x-ray scattering. The first order kinetics of the transformation are driven by a distortion of the ligand binding motifs. This reversible transformation of MSCs presents a missing bridge between molecular isomerization and solid-solid transformations.

The MSCs small size and high ligand/core ratio, gives them “softer” inter-particle interactions, with access to a richer phase diagram beyond the classical close packed structures seen with larger particles. We have recently found remarkable hierarchical assembly behavior of these MSC nanomaterials. These CdS MSCs can self-assemble into highly aligned structures, which span over six orders of magnitude in length scale. The MSCs assemble into filaments with hexagonal interparticle geometry, which bundle into larger fibers, and into centimeter-length superstructures of highly ordered thin films patterns. The thin films have long-range periodicity and interesting optical properties that emanate from the MSC core and/or the organic interconnections. The multiscale self-organization behavior of these MSC patterned films displays similarities to biosystems, providing a new platform for the design and study of materials.

References: JACS 140, 3652 (2018), Science 363, 731 (2019), Nat. Mat. (accepted 2022)

Halide perovskite semiconductors can merge the highly efficient operational principles of conventional inorganic semiconductors with the low‑temperature solution processability of emerging organic and hybrid materials, offering a promising route towards cheaply generating electricity as well as light. Following a surge of interest in this class of materials, research on halide perovskite nanocrystals (NCs) as well has gathered momentum in the last years. While most of the emphasis has been put on CsPbX₃ perovskite NCs, more recently the so-called double perovskite NCs, having chemical formula A⁺₂B⁺B³⁺X₆, have been identified as possible alternative materials, together with various other metal halides structures and compositions, often doped with various other elements. This talk will also discuss the research efforts of our group on these materials. I will highlight how for example halide double perovskite NCs are much less surface tolerant than the corresponding Pb-based perovskite NCs and that alternative surface passivation strategies will need be devised in order to further optimize their optical performance.

Organic-inorganic group 14 metal-halide hybrids have received extraordinary attention recently, owing to the remarkable electronic and photonic properties that three-dimensional (3D) lead-halide perovskites display, which have revolutionized the field of photovoltaics and light emission. This talk would explore our findings beyond the focus of the 3-D systems, highlighting instead the outstanding synthetic versatility of the more diverse low-dimensional (i.e. 2-D, 1-D, and 0-D) related family of the materials. In particular, the importance of the judicial choice of organic species in determining the formation of specific inorganic lattice architectures and/or supramolecular frameworks would be emphasized. The effects of molecular tuning on the physical properties of the overall materials, which manifest in the tailored performances of the corresponding solution-processed optoelectronic devices, such as solar cells, light-emitting diodes, and memory resistors would be covered. Future direction and opportunity in which the materials can play a role in technologically-relevant applications would also be discussed.

Lead halide perovskites (LHPs) have emerged as one of the most important class of materials for potential optoelectronics. Creative choice of ‘A’ site cation leads to the realization of structures having intriguing properties. LHPs with reduced dimensionality exhibit a strong light-matter interaction due to stronger quantum and dielectric confinements. We have investigated linear and non-linear optical properties in face-shared one-dimensional pyridinium lead iodide (PyPbI₃) single crystals. An efficient third-harmonic generation (THG) with a high laser-induced damage threshold (LIDT) is the highlight of this system. We observe a selective enhancement in THG for excitation at optical communication wavelength (~1.5 microns) corresponding to bandgap resonance. Strong exciton-phonon interaction results in highly Stokes-shifted self-trapped excitonic (STE) emission at the temperature range of 5-300 K. Temperature-dependent photoluminescence (PL) study reveals an interplay between anharmonicity and dynamic disorder leading to complex emission properties, which gets further complicated by a phase transition at 170 K. Our results shed light on the fundamental physics behind the complex carrier recombination process and provide an encouraging beginning to exploring similar 1D metal halides for potential application in non-linear optical photonic devices.

Colloidal semiconductor nanocrystals (NCs) are a class of nanomaterials that exhibit intriguing physical properties for application in optoelectronic devices such as solar cells, photodetectors, lasers, and light-emitting diodes (LEDs). Colloidal NCs are typically synthesized with wet-chemistry approaches that require further post-synthesis treatments to tune the properties of the obtained nanomaterial for the desired application. For example, the organic ligands that passivate the NC surface can be replaced by careful surface-engineering via ligands exchange reactions, and ad-hoc purification procedures must be employed to remove impurities in the NC solution which can tamper with the material in solid-state. Post-synthesis treatments are particularly relevant in the case of perovskite NCs CsPbBr₃ [1] to improve the photoluminescence quantum yield [2] and device performance [3]. Nevertheless, other types of colloidal NCs benefit from post-synthesis treatments leading to tailored device performance and increased stability [4].

Here, I will show how post-synthesis treatments can be employed to improve the performance of LEDs based on colloidal semiconductor NCs. In the case of perovskite NCs, I will discuss a novel purification process based on fractional freezing of solutions. In fact, we exploited the solubility differences between ligands and NC at low temperatures by precipitating the excess of organics freezing the NC dispersion at -20°C. The obtained LEDs demonstrated a noticeable improvement in performance upon fractional freezing of impurities, achieving an external quantum efficiency up to 8.9%, which is 3 times the maximum EQE obtained with the classical antisolvent approach (EQE_max = 2.4%).

Optical and electronic properties of metal oxide nanocrystals (MO NCs) strongly depend on the presence of depletion layers derived from the presence of surface states. In addition, MO NCs exhibit a localized surface plasmon resonance (LSPR), offering tunable features enabled by doping, both via electrochemical or photochemical charging. [1] Dynamic control over the LSPR makes MO NCs promising in optoelectronics and storage devices. [2] By manipulating the NCs depletion layer, it is possible to control their electronic properties. However, the mechanism behind this phenomenon is very complex, and not yet fully understood. [3] In particular, the tuning of several parameters, including the material under consideration, the size of the NCs, and the presence of multiple core-shell systems, enable the depletion layer engineering. To do this, it is possible to calculate the band and carrier density profiles for NCs with different features. In this work, a new framework has been introduced that can predict the behavior and physics under the MO NC photodoping process, revealing that the charging mechanism is unexpectedly based on the electronic rearrangement of the energy bands. Numerical simulations were experimentally supported by studying the case of a core-shell structure of Sn:In2O3/In2O3 NCs, by tuning the thickness of the shell, as well as post-synthetically, both by photodoping and reversible chemical reactions. The engineering of the depletion layer and the consequent manipulation of the electronic structure allows to significantly increase the sensitivity of LSPR and to target specific properties in MO NCs. The fine-tuning of the NCs band structure has enabled an improvement in charge storage capacity, which represents a step towards fully light-driven energy storage devices.

CoSe is considered as a promising non-noble-metal electrocatalyst for hydrogen evolution reaction (HER) in acid media. However, there is still space for improvement on this electrocatalyst, and one of the possible strategies is to combine it with small amount of noble metal. Therefore, we synthesized CoSe nanocrystals (NCs) decorated with Ru clusters. The Ru-CoSe nanocomposite shows better performance than the pristine CoSe NCs, and is even found to outperform the benchmark Pt/C catalyst under high current conditions. To find the reason behind the outstanding electrocatalytic performance of Ru-CoSe nanocomposite, we discover that this Ru−CoSe system undergoes chemical and structural transformations under hydrogen evolution conditions in acid media: the hexagonal CoSe NCs are converted into CoSe₂ NCs which have a cubic structure. The Co that is extracted from CoSe during this process combines with oxide and hydroxide species and forms a Co oxide/hydroxide layer; the Ru clusters aggregate and form Ru NCs. Such modifications result in a final Ru−CoOx/Co(OH)_x−CoSe₂ nanocomposite that exhibits an enhanced hydrogen evolution activity.

Photonic crystals (PhCs) are defined as composites made of media bearing different dielectric constant or refractive index ordered in a mono, bi or tridimensional periodic structure, and whose lattice constant is comparable to the wavelength of visible or near-infrared light. PhCs are nowadays exploited in functional architecture, enhancement of photon absorption for photovoltaic cells, emission control, lasing, and sensing.[1]

Recently, fabrication of polymer planar photonic crystals from polymer solutions has been receiving particular attention thanks to the ease of tuning of their properties and of scaling their fabrications on large areas.[2] Although major improvements of their performances were made through sophisticated engineering of their structure and properties, solution processed polymer structures still cannot compete with vacuum-deposited inorganic systems which are capable of reaching higher dielectric contrasts, strongly sought-after for their applicability.[3] On the other hand, these fabrications are hardly scalable and extremely costly.

We will demonstrate an alternative way to prepare highly inorganic transparent hybrid thin films of titania (TiO₂) and silica (SiO₂) in presence of a polymer stabilizer via spin-coating including comparison with similar structures from literature. Fabrication of films is achieved by exploiting sol-gel like in situ reactions between inorganic matrices from alkoxide precursors and polymer additive. The new titania based hybrid shows the highest refractive index reported in the literature for solution processed films leading to the largest value of dielectric contrast for solution processable planar PhCs. Moreover, the design of the hybrids allows for mutual processability with 0D colloidal nanocrystals solutions in the fabrication of optical microcavities for emission control and lasing applications.[4] Some examples will be reported and discussed.

CdSe/CdS core-crown nanoplatelets display minimal shifts in optical features relative to their respective CdSe core, but yield higher stability and photoluminescence quantum efficiencies,[1] imperative for cutting-edge photonic applications including lasing. In the present work, using femtosecond pump-probe spectroscopy we assessed the influence of crown (CdS) lateral area on the optical gain threshold, gain lifetime and gain bandwidth of CdSe/CdS core-crown nanoplatelets. Our results demonstrate that thin CdS crowns lowers the gain threshold twofold compared to the CdSe core, achieving gain close to 1 exciton per nanoplatelet. The lower gain thresholds for the thin-crown nanoplatelets is likely a consequence of efficient surface trap passivation, as we also observed an increasing gain lifetime of 700 ps, nearly threefold longer than CdSe cores for similar exciton densities. Further increase of the crown lateral area increased the gain threshold to an exciton density similar to the core nanoplatelets, yet the threshold is obtained at a fourfold lower photon flux, as a result of higher absorption cross section. The gain lifetime of 400 ps is also still twofold longer than core nanoplatelets. Furthermore, the thick crown broadened the gain bandwidth to 105 nm. The maximum material gain obtained from the core and core-crown nanoplatelets are similar and reach 15000 cm^-1.  Our studies confirm that core-crown nanoplatelets are promising materials for light amplification.

Superparticles made from colloidal nanocrystals have recently shown great promise in bridging the nanoscale and mesoscale, building artificial materials with properties designed from the bottom-up. As these properties depend on the dimension of the superparticle, there is a need for a general method to produce monodisperse nanocrystal superparticles. Here, we demonstrate an approach that readily yields spherical nanocrystal superparticles with a polydispersity as low as 2%. We show that this strategy is general and rapid, yielding monodisperse superparticles with controllable sizes and morphologies, including core/shell structures, within a few minutes. The superparticles show a high optical quality that results in lasing through the whispering gallery modes of the spherical structure, with an average quality factor of 1600. Assembling superparticles into small clusters selects the wavelength of the lasing modes, demonstrating an example of collective photonic behavior using these artificial solids.

2D materials such as chemically modified graphenes, transition metal dichalcogenides, layered double hydroxide to name only a few, are having a huge impact on electrocatalysis providing materials with outstanding activity for a variety of reactions.[1] However, despite the intense research efforts in this field, a clear identification of the real active sites in many reactions remains a great challenge, given the necessity to employ spatially and structurally sensitive techniques in operando conditions (i.e. during the application of an electrochemical potential in the presence of an electrolyte). Recently, we have developed an innovative approach to the study of 2D materials by using electrochemical Scanning tunneling microscopy. As demonstrated by a seminal paper,[2] this technique allows identifying catalytic processes at the nanoscale by observing a typical noise in the tunneling current, which is due to instantaneous variations of the tunneling junction. Starting from here, we have introduced a new quantity, the tunneling current roughness, which allowed us to acquire quantitative measurements of the electrocatalytic activity with subnanometric precision.

By using special model systems consisting of CVD grown transition metal dichalcogenides thin films (MoSe₂ and WSe₂), and iron ultrathin films covered by graphene, we achieved even atomic resolution in operando during the hydrogen evolution reaction. This allowed us to identify and quantitatively compare the chemical activity of several chemical and morphological features such as single atom vacancies, Fe-C4 defects, step edges, and even exotic line defects such as metallic twin boundaries.[3] In particular we could determine that iron single atoms trapped within the graphene basal plane are even more active than platinum, the benchmark catalyst for the hydrogen evolution reaction.[4]

In recent years, a plethora of material systems have been designed and prepared to increase the performance of light harvesting and light-emitting technologies, and to develop new and attractive applications. Limitations of state-of-the-art devices based on organics (both conjugated polymers or small molecules/oligomers) derive largely from material stability issues after prolonged operation. This challenge could be tackled by leveraging the enhanced stability of carbon nanostructures, including carbon nanotubes, nanoribbons and the large family of graphene based materials, in carefully designed nano-hybrid or nano-composite architectures to be integrated within photo-active layers, paving the way to the exploitation of these materials in contexts in which their potential has not been yet fully revealed. In this talk, we discuss the theoretical background behind carbon nanomaterials hybridization with other materials for the establishment of novel optoelectronic properties. By retrieving to a multiscale computational protocol, it is possible to assess the opto-electronic, transport and transfer properties of the assembly and its components, in order to optimize the absorption of light and the transfer of energy/charges at the interface. In particular, we will focus on 0D/2D interfaces in which graphene is coupled with small organic molecules, and how the creation of an hybrid system is beneficial for the enhancement of the optoelectronic and transport properties of the materials.

Curved π-conjugated polycyclic hydrocarbons (or nanographenes) has become an important research targets owing to their fascinating intermolecular packing and extraordinary chiraloptical properties resulting from their contorted conformation. In general, two distinct approaches have been established for the synthesis of curved nanographenes: one is the incorporation of steric strain in their periphery, the other is to introduce the non-hexagonal rings (i.e. pentagon, heptagon, octagon) in their skeleton which induce the nonplanar nature. The resultant curvature in a π-conjugated system often yields an unusual electronic structure and unprecedented physical properties. Here, I will talk the reasonable synthesis of several curved nanographenes and graphene nanoribbons with different topologies, such as saddle-shaped and wavy-shaped open-shell radicaloids,^{[1], [2]} azulene-embedded helical nanographenes,^[3] and curved graphene nanoribbons with multiple edge structures.^[4],[5] Apart from the synthetic strategies, the structure-property relations of these π-systems as well as their optical, electronic and magnetic properties will be also presented. Our work provides a new insight into the synthesis of functional curved aromatics as well as their potential applications in nanoelectronic and spintronic devices.