Nanostructured Materials

Organic Photovoltaics

We study all organic, non-fullerene solar cells. Establishing how the conformation of organic photovoltaic (OPV) polymers affects their electronic and transport properties is critical in order to determine design rules for new OPV materials and in particular to understand the performance enhancements recently reported for ternary blends. We carried out coupled classical and ab initio molecular dynamics simulations showing that polymer linkage twisting significantly reduces optical absorption efficiency, as well as hole transport rates in donor polymers. We predicted that blends with components favoring planar geometries contribute to the enhancement of the overall efficiency of ternary OPVs. Furthermore, our electronic structure calculations for the PTB7–PID2–PC71BM system showed that hole transfer rates are enhanced in ternary blends with respect to their binary counterpart. Our results also pointed at thermal disorder in the blend as a key reason responsible for device voltage losses and at the need to carry out electronic structure calculations at finite temperature to reliably compare with experiments.

• "Mesomeric control of the optoelectronic properties of polymerized small molecule acceptors", Diego Sorbelli, Yilei Wu, Zhenan Bao, and Giulia Galli, J. Mater. Chem. A 12, 25837-25849 (2024).
• "Tuning polymer-backbone coplanarity and conformational order to achieve high-performance printed all-polymer solar cells", Yilei Wu, Yue Yuan, Diego Sorbelli, Christina Cheng, Lukas Michalek, Hao-Wen Cheng, Vishal Jindal, Song Zhang, Garrett LeCroy, Enrique Gomez, Scott Milner, Alberto Salleo, Giulia Galli, John Asbury, Michael Toney, and Zhenan Bao, Nat. Commun. 15, 2170 (2024).
• "Twisted A-D-A Type Acceptors with Thermally-Activated Delayed Crystallization Behavior for Efficient Nonfullerene Organic Solar Cells", Yilei Wu, Sebastian Schneider, Yue Yuan, Ryan M. Young, Tommaso Francese, Iram F. Mansoor, Peter J. Dudenas, Yusheng Lei, Enrique D. Gomez, Dean M. DeLongchamp, Mark C. Lipke, Giulia Galli, Michael R. Wasielewski, John B. Asbury, Michael F. Toney, and Zhenan Bao, Adv. Energy Mater. 12, 2103957 (2022).
• "Intra-molecular Charge Transfer and Electron Delocalization in Non-Fullerene Organic Solar Cells", Qingh Wu, Donglin Zhao, Matthew Goldey, Alexander Filatov, Valerie Sharapov, Yamil Colon, Zhengxu Cai, Wei Chen, Juan de Pablo,Giulia Galli and Luping Yu, ACS Applied Materials & Interfaces 10, 10043-10052 (2018).
• "Planarity and multiple components promote organic photovoltaic efficiency by improving electronic transport ", Matthew Goldey, Daniel Reid, Juan J. de Pablo, and Giulia Galli, Phys. Chem. Chem. Phys. 18, 31388-31399 (2016).

Thermally activated delayed fluorescent compounds

We investigate all organic thermally activated delayed fluorescence (TADF) emitter in the gas phase- and in the high-packing fraction limits at finite temperature, by combining first principles molecular dynamics with a quantum thermostat to account for nuclear quantum effects (NQE). Our results show a weak dependence of the singlet–triplet energy gap (ΔE_ST) on temperature in both the solid and the molecule, and a substantial effect of packing. Our findings highlight the importance of considering thermal fluctuations and NQE to obtain robust predictions of the electronic properties of TADF and to interpret experimental results.

• "Interplay of molecular dynamics and radiative decay of a TADF emitter in a glass-forming liquid", John R. Swartzfager, Gary Chen, Tommaso Francese, Giulia Galli, and John B. Asbury, Phys. Chem. Chem. Phys. 25, 3151-3159 (2023).
• "Quantum simulations of thermally activated delayed fluorescence in an all-organic emitter", Tommaso Francese, Arpan Kundu, Francois Gygi, and Giulia Galli, Phys. Chem. Chem. Phys. 24, 10101 (2022).

Core-Shell Nanoparticles and Nanoplatelets

We carried out first principles calculations of the optoelectronic properties of In_xP_z and In_xGa_yP_z QDs interfaced with zinc chalcogenide shells and showed that fine-tuning the composition of the core is critical to achieving narrow emission lines. We showed that core–shell nanoparticles, where the core has the same diameter but different stoichiometries, may absorb and emit at different wavelengths, leading to broad absorption and emission spectra. The value of the fundamental gap of the core–shell particles depends on the ratio between the number of group III and P atoms in the core and is maximized for the 1:1 composition. We also found that the interplay between quantum confinement and strain determines the difference in the electronic properties of III–V QDs interfaced with ZnS or ZnSe shells. The effect of strain was also evident in a separate study on CdSe nanoplatelets, which aimed to identify the factors determining nanoplatelet photophysical properties. We employed many body perturbation theory at the GW level and solved the Bethe-Salpeter equation to obtain absorption spectra and excitonic properties which led to a model that disentangles the effects of quantum confinement, strain induced by passivating ligands, and dielectric environment on the electronic properties of nanoplatelets.

• "Determining the Structure–Property Relationships of Quasi-Two-Dimensional Semiconductor Nanoplatelets", Arin Greenwood, Sergio Mazzotti, David Norris and Giulia Galli, J. Phys. Chem. C, 125, 8, 4820–4827 (2021)
• "Stoichiometry of the Core Determines the Electronic Structure of Core-Shell III–V/II–VI Nanoparticles", Mariami Rusishvili, Stefan Wippermann, Dmitri Talapin and Giulia Galli, Chem. Mater. 32, 22, 9798-9804 (2020)

Buried interfaces in all-inorganic nanocrystalline solids

Controlling the surface chemistry of nano building blocks and their interfaces with ligands is one of the outstanding challenges for the rational design of all-inorganic nanostructured solids. We carried out a combined theoretical and experimental study to characterize, at the atomistic level, buried interfaces in solids of InAs nanoparticles capped with Sn₂S₆₄- ligands. These prototypical nanocomposites are known for their promising transport properties and unusual negative photoconductivity. We found that inorganic ligands dissociate on InAs to form a surface passivation layer. A nanocomposite with unique electronic and transport properties is formed, that exhibits type II heterojunctions favourable for exciton dissociation. We identified how the matrix density, sulfur content and specific defects may be designed to attain desirable electronic and transport properties, and we explain the origin of the measured negative photoconductivity of the nanocrystalline solids.

• "Surface Chemistry and Buried Interfaces in All-Inorganic Nanocrystalline Solids", Emilio Scalise, Vishwas Srivastava, Eric M. Janke, Dmitri Talapin, Giulia Galli, and Stefan Wipperman, Nature Nanotech. 13, 841-848 (2018)

Chalcogenide nanoparticles: solids and embeddings

Lead chalcogenide nanoparticle solids have been successfully integrated into certified solar cells and represent promising platforms for the design of novel photoabsorbers for photoelectrochemical cells. The role of interactions between nanoparticles is not yet well-understood. Using first-principles molecular dynamics and electronic structure calculations, we investigated the combined effect of temperature and interaction on functionalized lead chalcogenide nanoparticles (NPs). We showed that at finite temperature, interacting NPs are dynamical dipolar systems, with the average values of dipole moments and polarizabilities substantially increased with respect to those of the isolated building blocks.

We also carried out atomistic calculations on chalcogenide nanostructured materials, i.e., PbSe QDs in CdSe matrices and CdSe embedded in PbSe, and we established how interfacial and core structures affect their electronic properties. We showed that defects present at interfaces of PbSe nanoparticles and CdSe matrices give rise to detrimental intragap states, degrading the performance of photovoltaic devices. Instead, the electronic gaps of the inverted system (CdSe dots in PbSe) are clean, indicating that this material has superior electronic properties for solar applications.

• "Modelling superlattices of dipolar and polarizable semiconducting nanoparticles", Sergio Mazzotti, Federico Giberti, and Giulia Galli, Nano Letters 19(6), 3912-3917 (2019)
• "Emergent electronic and dielectric properties of interacting nanoparticles at finite temperature", Arin R. Greenwood, Márton Vörös, Federico Giberti, and Giulia Galli, Nano Letters 18, 255 (2018)
• "Colloidal Nanoparticles for Intermediate Band Solar Cells", Márton Vörös, Giulia Galli, and Gergely Zimanyi , ACS Nano 9, 6882 (2015)
• "Design of heterogeneous chalcogenide nanostructures with pressure-tunable gaps and without electronic trap states", Federico Giberti, Márton Vörös, and Giulia Galli, Nano Lett. 17 (4), pp 2547–2553 (2017)

Embedded Si nanocrystals

Si nanocrystals (NCs) are often synthesized in oxide or nitride matrices. Our coupled classical and quantum simulations of 1-2 nm Si nanoparticles embedded in amorphous-SiO₂ has shown that by tuning the density of the oxide , one may form nanoscale heterojunctions with either straddling (type I) or staggered (type II) energy gaps. We have also found that interfacial strain plays a key role in determining the variation of the nanoparticle gap as a function of size, as well as conduction band offsets. NCs extracted from matrices with their oxide shell have allowed us to study the origin of blinking. In addition to oxides we have studied Si NPs in ZnS matrices and found that upon high temperature amorphization of the host chalcogenide, sulfur atoms are drawn to the NP surface. Sulfur content may be engineered to form a type II heterojunction, with complementary charge transport channels for electrons and holes.

• "Surface Dangling Bonds Are a Cause of B-Type Blinking in Si Nanoparticles", Nicholas Brawand, Márton Vörös and Giulia Galli, Nanoscale 7, 3737 (2015)
• "Solar nanocomposites with complementary charge extraction pathways for electrons and holes: Si embedded in ZnS", Stefan Wippermann, Márton Vörös, Adam Gali, Francois Gygi, Gergely Zimanyi and Giulia Galli, Phys. Rev. Lett. 112, 068103 (2014)
• "Tailored nanoheterojunctions for optimized light emission", T. Li, F. Gygi and G. Galli, Phys. Rev. Lett. 107, 206805 (2011)

Ligand engineering

Band edge positions of semiconductors determine their functionality in many optoelectronic applications such as photovoltaics, photoelectrochemical cells, and light emitting diodes. Here we show that band edge positions of lead sulfide (PbS) colloidal semiconductor nanocrystals, specifically quantum dots (QDs), can be tuned over 2.0 eV through surface chemistry modification. We achieved this remarkable control through the development of simple, robust, and scalable solution-phase ligand exchange methods, which completely replace native ligands with functionalized cinnamate ligands, allowing for well-defined, highly tunable chemical systems. By combining experiments and ab initio simulations, we establish clear relationships between QD surface chemistry and the band edge positions of ligand/QD hybrid systems. We find that in addition to ligand dipole, inter-QD ligand shell inter-digitization contributes to the band edge shifts. We expect that our established relationships and principles can help guide future optimization of functional organic/inorganic hybrid nanostructures for diverse optoelectronic applications.

• "Designing Janus Ligand Shells on PbS Quantum Dots using Ligand-Ligand Cooperativity", Noah D. Bronstein, Marissa Martinez, Daniel M. Kroupa, Márton Vörös, Haipeng Lu, Nicholas P. Brawand, Arthur J. Nozik, Alan Sellinger, Giulia Galli, Matthew C. Beard, ACS Nano, 13(4), 3839-3846 (2019)
• "Optical Absorbance Enhancement in PbS QD/Cinnamate Ligand Complexes", Daniel M. Kroupa, Márton Vörös, Nicholas P. Brawand, Noah Bronstein, Brett McNichols, Chloe Castaneda, Arthur Nozik, Alan Sellinger, Giulia Galli and Matthew Beard, J. Phys. Chem. Lett., 9(12) 3425-3433 (2018)
• "Tuning Colloidal Quantum Dot Band Edge Positions through Solution-Phase Surface Chemistry Modification", Daniel M. Kroupa*, Márton Vörös*, Nicholas P. Brawand, Brett W. McNichols, Jing Gu, Elisa M. Miller, Arthur J. Nozik, Alan Sellinger, Giulia Galli, Matthew C. Beard, Nature Comms., 8, 15257 (2017) (*These authors contributed equally to this work)

Defect states and charge transport in quantum dots

The presence of trap states in the electronic gap of semiconducting limits their usability in solar cells, and developing a universal strategy to remove trap states is a persistent challenge. Using calculations based on density functional theory, we studied defects states in hydrogenated Si dots and in lead chalcogenide nanoparticles. We showed that hydrogen acts as an amphoteric impurity on PbS nanoparticle surfaces, passivating defects arising from ligand imbalance or off-stoichiometric surface terminations. Using constrained density functional theory calculations, we showed that hydrogen treatment of defective nanoparticles is also beneficial for charge transport in films. The same techniques was used to study shallow and deep impurity states in Si nanoparticles.

• "Charge Transport in Nanostructured Materials: Implementation and Verification of Constrained Density Functional Theory", Matthew Goldey, Nicholas Brawand, Márton Vörös, and Giulia Galli, J. Chem. Theory Comput., 13(6), 2581-2590 (2017)
• "Hydrogen treatment as a detergent of electronic trap states in lead chalcogenide nanoparticles", Márton Vörös, Nicholas Brawand, and Giulia Galli, Chem. Mater. 29(6), pp 2485–2493 (2017)
• "Defect states and charge transport in quantum dot solids", Nicholas Brawand, Matthew Goldey, Márton Vörös, and Giulia Galli, Chem. Mater., 29(3), pp. 1255-1262 (2017)

Novel Silicon Phases

Silicon exhibits a large variety of different bulk phases, allotropes and composite structures, such as e. g. clathrates or nanostructures, at both higher and lower densities compared to diamond-like Si-I. New Si structures continue to be discovered. These novel forms of Si offer exciting prospects to create Si based materials, that are non-toxic and earth-abundant, with properties tailored precisely towards specific applications, including solar energy conversion devices.

• "Novel Silicon phases and nanostructures for solar energy conversion", Stefan Wippermann, Yuping He, Marton Voros, and Giulia Galli, Appl. Phys. Rev. 3, 040807 (2016)
  Work highlighted in Exotic forms of silicon, P. Craig Taylor, Phys. Today 2016
• "Surface Dangling Bonds Are a Cause of B-Type Blinking in Si Nanoparticles", Nicholas Brawand, Márton Vörös and Giulia Galli, Nanoscale 7, 3737 (2015)

Excitation Spectra of Si and Ge Nanoparticles

We carried out density functional and many body perturbation theory calculations of the electronic, optical, and impact ionization properties of Si and Ge nanoparticles (NPs), including core structures based on high-pressure bulk Si and Ge phases. Si particles with a BC8 core structure exhibit significantly lower optical gaps and multiple exciton generation (MEG) thresholds, and an order of magnitude higher MEG rate than diamondlike ones of the same size. Hence BC8 nanoparticles may be promising candidates for MEG-based solar energy conversion. High pressure structures of Ge nanoparticles (ST12) show as well promising MEG properties.

• "Ab initio optoelectronic properties of silicon nanoparticles: Excitation energies, sum rules, and Tamm-Dancoff approximation", Dario Rocca, Márton Vörös, Adam Gali and Giulia Galli, J. Comp. Theor. Chem. 10, 3290 (2014)
• "Germanium nanoparticles with non-diamond core structures for solar energy conversion", Márton Vörös, Stefan Wippermann, Bálint Somogyi, Adam Gali, Dario Rocca, Giulia Galli, and Gergely T. Zimanyi, J. Mater. Chem. A 2, 9820 (2014)
• "High-Pressure Core Structures of Si Nanoparticles for Solar Energy Conversion", S. Wippermann, M. Vörös, D. Rocca, A. Gali, G. Zimanyi and G. Galli, Phys. Rev. Lett. 110, 046804 (2013)
• "Increasing impact ionization rates in Si nanoparticles through surface engineering: A density functional study", M. Vörös, D. Rocca, G. Galli, G.T. Zimanyi and A. Gali, Phys. Rev. B 87, 155402 (2013)
• "High energy excitations in silicon nanoparticles", A. Gali, M. Vörös, D. Rocca, G. Zimanyi and G. Galli, Nano Lett. 9, 3780 (2009)

MoS₂ Nanoparticles

Bulk MoS2, a prototypical metal dichalcogenide, is an indirect band gap semiconductor with negligible photoluminescence. When the MoS2 crystal is thinned to a monolayer, a strong photo-luminescence emerges, indicating an indirect to direct band gap transition, as predicted by ab-initio calculations. This observation shows that quantum confinement in layered d-electron materials such as MoS2 may provide new opportunities for engineering the electronic structure of matter at the nanoscale.

• "Emerging Photoluminescence in Monolayer MoS2", A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C.-Y. Chim, G. Galli and F. Wang, Nano Lett. 10, 1271 (2010)
• “Electronic properties of MoS2 nanoparticles”, T. Li and G. Galli, J. Phys. Chem. C 111, 16192 (2007)

Si and Si/Ge Nanostructures

We investigate a number of Silicon and Silicon/Germanium nanostructures using first-principles calculations. We studied the thermoelectric properties of a newly synthesized Si-based ternary clathrate K8Al8Si38, composed of ∼1 nm hollow cages with a metal atom inside. We found that, similar to other nanostructured type I clathrates, this system is a semiconductor and has a low thermal conductivity (∼1 W/mK), and that the cage structural disorder induced by atomic substitution plays a crucial role in determining the conductivity.

We also studied a number of properties of Si and SiGe nanowires, investigating how morphology affects the changes to thermal conductivity at the nanoscale. We found that the computed thermal conductivity strongly depends on the surface structure and defects present in the bulk of the nanowires. Our results were used to rationalize several experiments showing strong reduction of the conductivity in Si nanowires. We also computed the thermal conductivity of planar superlattices, arrays of Ge nanowires, and nanodots embedded in Si, by using a fully atomistic Monte Carlo solution of the Boltzmann transport equation, and we investigated how dimensionality affects heat transport in Si-Ge superlattices.

Finally, we performed molecular dynamics and lattice dynamics calculations of nanoporous silicon to show that the thermal conductivity may attain values 10−20 times smaller than in bulk Si for porosities and surface-to-volume ratios similar to those obtained in recently fabricated nanomeshes.

• "Nanostructured Clathrate Phonon Glasses: Beyond the Rattling Concept", Yuping He and Giulia Galli, Nano Lett. 14, 2920 (2014)
• "Heat Transport in Amorphous Silicon: Interplay Between Morphology and Disorder", Y. He, D. Donadio and G. Galli, Appl. Phys. Lett. 98, 144101 (2011)
• "Thermal Transport in Nanoporous Silicon: Interplay between disorder at the mesoscopic and the atomic scale", Y. He, D. Donadio, J.-H. Lee, J. C. Grossman and G. Galli, ACS Nano 5, 1839 (2011)
• "Silicon stops heat in its tracks", G. Galli and D. Donadio, Nat. Nanotech. 5, 701 (2010)
• "Dimensionality and heat transport in Si-Ge superlattices", I. Savic, D. Donadio, F. Gygi and G. Galli, Appl. Phys. Lett. 102, 073113 (2013)
• "Morphology and Temperature Dependence of The Thermal Conductivity of Nanoporous SiGe", Y. He, D. Donadio and G. Galli, Nano Lett. 11, 3608-3611 (2011)
• "Cluster expansion and optimization of thermal conductivity in SiGe nanowires", M. K. Y. Chan, J. Reed, D. Doonadio, T. K. Mueller, Y. S. Meng, G. Galli and G. Ceder, Phys. Rev. B 81, 174303 (2010)
• "Microscopic origin of the reduced thermal conductivity of silicon nanowires", Yuping He and Giulia Galli, Phys. Rev. Lett. 108, 215901 (2012)
• "Temperature dependence of the thermal conductivity of thin silicon nanowires" D. Donadio and G. Galli, Nano Lett. 10, 847 (2010)
• "Atomistic simulations of heat transport in silicon nanowires", D. Donadio and G. Galli, Phys. Rev. Lett. 102, 195901 (2009)
• Ab-initio calculations of absorption spectra of semiconducting nanowires within many body perturbation theory, Y. Ping, D. Rocca, D. Lu and G. Galli, Phys. Rev. B 85, 035316 (2012)