About

Shabnam Akhtari is a professor at the Pennsylvania State University, based in the 339 McAllister Building. Her research interests include Number Theory, Geometry of Numbers, and Diophantine Analysis. Her work focuses on these areas, contributing to the understanding of their underlying mathematical structures and properties.

Research topics

• Computer Science
• Materials science
• Optoelectronics
• Physics
• Optics
• Nanotechnology
• Artificial Intelligence
• Engineering physics
• Chemistry
• Molecular physics
• Condensed matter physics
• Chemical physics

Selected publications

• Resolving State-Specific Energy Flow in Metal Nanoclusters Using 2D Electronic Spectroscopy

  The Journal of Physical Chemistry Letters · 2026-03-10

  articleOpen accessSenior authorCorresponding

  Sub-to-few-nanometer gold nanoclusters exhibit a manifold of electronic states that result in nanocluster- and state-specific mechanisms of energy flow, which present new opportunities for developing photonic materials. Due to the spectral congestion of conventional ultrafast transient methods, mechanistic insights into energy flow are difficult to achieve for these systems. The use of two-dimensional electronic spectroscopy (2DES) to resolve electronic relaxation dynamics with state specificity for metal nanoclusters is described, along with prospects for future research. Excitation-detection frequency correlations inherent to 2D measurements resolve the electronic relaxation within specific gold superatom states. The state specificity of 2DES is extended to distinguish the influences of the electronic state symmetry on carrier relaxation using polarization-dependent measurements. Crosspeak-specific 2DES has also been used to distinguish sequential relaxation through nondegenerate electronic state manifolds of nanoclusters from the collective dynamics of metallic nanoparticles. These results demonstrate the power of 2DES for aiding the understanding of metal nanocluster photophysical properties.

  PublisherDOI

• Diverse Superatomic Magnetic and Spin Properties of Au₁₄₄(SC₈H₉)₆₀ Clusters

  ACS Central Science · 2025-05-29 · 8 citations

  articleOpen accessSenior authorCorresponding

  Au144(SC8H9)60, a colloidal cluster with a 1.7 nm inorganic diameter, exhibits both metallic and molecular-like behavior, along with a distribution of unfilled superatom states. Its 1.7–2.5 eV electronic transitions were probed with variable-temperature, variable-field magnetic circular dichroism (VTVH⇀-MCD), revealing two energy regions with distinct responses. Below 2.0 eV, MCD transitions exhibited diverse VTVH⇀ responses, including both paramagnetic and diamagnetic behavior, implicating multiple nondegenerate initial states originating within the open-shell superatom S, D, and H HOMO manifold. Above 2.0 eV, uniform field-dependent responses suggested spin-vibronic coupling due to metal–ligand mixing. The Au144(SC8H9)60 magneto-optical response is surprisingly complex given the system’s high electronic-state density; discrete structural domains of the cluster, including the superatomic metal core, likely contribute to this diversity. These results show the potential to investigate and tailor the magneto-optical and spin properties of these clusters through structurally precise synthesis and also identify superatomic colloids as candidates for advancing spin-based technologies.

  PublisherDOI

• Structure-Dependent Electronic Relaxation Dynamics of Two-Dimensional Silver Monolayers

  Nano Letters · 2025-12-01 · 3 citations

  articleSenior authorCorresponding

  The electronic relaxation dynamics of two-dimensional silver polar metal heterostructures (2D-PMets), isolated with two different Ag lattice structures, were studied with femtosecond transient absorption (fs-TA) spectroscopy. The two 2D Ag phases, called Ag(1) and Ag(2), differ in atomic packing density, which leads to phase-specific ultralow frequency (ULF) phonon modes and visible electronic absorption transitions. Time-resolved kinetic traces for both phases were fit to a biexponential decay function, with the first decay component pertaining to ultrafast electronic relaxation and the second corresponding to carrier-phonon scattering. The first decay time constant τ1 is <400 fs for both phases. In contrast, carrier-phonon scattering exhibited lattice-specific and excitation wavelength-independent relaxation time constants; τ2 ∼ 2 ps for Ag(1) and ∼ 1 ps for Ag(2). The shorter τ2 in Ag(2) is attributed to increased carrier-phonon scattering probability in more close-packed lateral structures. The results indicate that atomic-level structure controls energy flow in spatially confined 2D materials.

  PublisherDOI

• Defect-Mediated Phase Engineering of 2D Ag at the Graphene/SiC Interface

  ArXiv.org · 2025-11-10 · 1 citations

  preprintOpen access

  Atomically thin silver (Ag) films offer unique opportunities in plasmonic, quantum optics, and energy harvesting, yet conventional growth methods struggle to achieve structural control at the monolayer limit. Here, we demonstrate phase-selective synthesis of large-area, crystalline 2D Ag films via defect-engineered confinement heteroepitaxy (CHet) at the epitaxial graphene/silicon carbide (EG/SiC) interface. By tuning graphene growth and post-growth defect introduction, two distinct Ag phases are achieved with disparate properties: a nearly commensurate Ag(1) lattice stabilized by vacancy and line defects in epitaxial graphene, and a denser Ag(2) phase preferentially grown with sp3-rich zero-layer graphene. Structural and spectroscopic characterization confirm lattice registry with the SiC substrate, while theoretical calculations reveal a thermodynamic preference for Ag(2) but an easier nucleation for Ag(1). Both phases are found to be semiconducting, with the Ag(2) phase exhibiting slightly enhanced n-doping of graphene. Notably, nonlinear optical measurements reveal a three-order magnitude difference in second-order susceptibility between the two phases, demonstrating promise for phase-tunable 2D metals in reconfigurable optoelectronic and metamaterial platforms.

  PublisherOA PDFDOI

• Understanding relaxation dynamics in structurally precise metal clusters using 2DES

  2025-09-16

  article1st authorCorresponding

  Two- and Three-dimensional electronic spectroscopy (2DES and 3DES) was used to study electronic energy relaxation in colloidal metal nanoparticle ensembles. Following plasmon excitation, these systems dissipate energy by sequential plasmon dephasing, “hot” electron-electron scattering, electron-phonon scattering and energy transfer to the environment. Using plasmon-resonant 2DES, hot electron dynamics were distinguished from thermalized carrier cooling in plasmonic nanorods. Polarization-dependent 2DES was used to study the importance of electronic state symmetry in carrier relaxation processes. Finally, 3DES was used to resolve electronic-state and phonon-mode-specific electron-phonon scattering.

  PublisherDOI

• Plasmon-mediated nonlinear optics and dynamics

  The Journal of Chemical Physics · 2025-11-05

  articleOpen access1st authorCorresponding
  PublisherOA PDFDOI

• Electronic Relaxation Dynamics of the Au ₄₂ (SC ₈ H ₉ ) ₃₂ Cluster Nanorod Studied Using 2D Electronic Spectroscopy

  The Journal of Physical Chemistry Letters · 2025-12-26 · 1 citations

  articleSenior authorCorresponding

  The rod-like Au42(SC8H9)32 monolayer-protected cluster (MPC) was studied using two-dimensional electronic spectroscopy (2DES). This study combined analysis of the excitation power, cross-peak specific maps, and time-dependent 2DES signals that resulted from excitation of a longitudinal electronic resonance at 13 500 cm–1. The Au42(SC8H9)32 longitudinal resonance is of interest due to the exceptional photothermal efficiency of this MPC. A traditional plasmonic gold nanorod is used throughout as a point of comparison. The excitation power study and 2DES results obtained from excitation of the longitudinal resonance were distinct from plasmonic excitations, thus implicating it as an exitonic system. The time-dependent signal amplitudes of 2DES cross-peaks showed that the longitudinal mode consisted of multiple electronic fine structure states that internally converted in a state-to-state manner. Taken together, these results point to a manifold of nondegenerate electronic states, rather than a collective plasmon resonance, that comprise the longitudinal mode excitation and dynamics of Au42(SC8H9)32.

  PublisherDOI

• Solvent Dependence of [Au₁₁(BINAP)₄X₂: X = Cl or Br]⁺ Cluster Electronic and Optical Properties

  The Journal of Physical Chemistry A · 2025-07-10 · 3 citations

  articleSenior authorCorresponding

  Complex cluster–solvent interactions were investigated for 11-metal-atom monolayer-protected gold nanoclusters dispersed in a variety of solvents. Ultraviolet–visible (UV–vis), photoluminescence, and femtosecond transient absorption (fs-TA) spectroscopies were employed to examine the effect of solvent identity on the photophysical properties of [Au11(BINAP)4X2]+, where X represents Cl or Br. UV–vis absorption spectra showed a narrowing of spectral peaks when the clusters were dispersed in protic solvents, indicating increased cluster rigidity in these solvents. Photoluminescence studies revealed that emission profiles can be tuned by modulating the cluster–solvent interaction. fs-TA experiments further supported the assignment of distinct radiative decay pathways based on the strength of the cluster–solvent interactions and also demonstrated that ethanol binding to [Au11(BINAP)4X2]+ clusters is more thermodynamically stable than butanol binding. 1H nuclear magnetic resonance spectroscopy provided evidence for cluster–solvent hydrogen bonding between [Au11(BINAP)4Cl2]+ and ethanol in the form of downfield-shifted and exchange-broadened peaks. These data indicate that cluster–solvent hydrogen bonds can be tuned, emphasizing the important role of the solvation shell in determining the electronic and optical properties of atomically precise nanoclusters.

  PublisherDOI

• The Influence of Passivating Ligand Identity on Au₂₅(SR)₁₈ Spin-Polarized Emission

  The Journal of Physical Chemistry Letters · 2025-05-15 · 3 citations

  articleSenior authorCorresponding

  Magnetic circular photoluminescence (MCPL) spectra were collected following 3.1 eV excitation of two ligand-passivated Au25(SR)18 monolayer-protected clusters (MPCs). Both clusters generated spin-polarized emission; however, the degree of circular polarization noted for Au25(SC8H9)18, which was passivated with the aromatic phenylethanethiol ligand, was 5× that obtained for Au25(SC3)18, whose passivating ligand was aliphatic. Variable-magnetic field data were analyzed to determine Landé g-factors and spectroscopic term symbols for observable transitions contributing to the clusters’ MCPL spectra. For Au25(PET)18, transitions originated from one doublet and two quartet fine-structure superatomic electronic states; by comparison, the Au25(SC3)18 spectrum contained only two components, both of which arose from doublet superatomic electronic states. Additionally, Faraday B-term contributions, which report on field-induced mixing, were more pronounced for Au25(SC3)18 spectral components. Therefore, the decreased spin-polarized emission by Au25(SC3)18 was attributed to stronger coupling to nonradiative decay channels. These results suggest the Au25(SR)18 cluster’s passivating ligand can be used to tune the relative populations of emissive fine-structure states, the extent of mixing between radiative and nonradiative states, and the amplitude of spin-polarized emission in MPCs.

  PublisherDOI

• Understanding Nanoparticle Electronic Spin‐State Dynamics and Properties Using Variable‐Temperature, Variable‐Field Magnetic Circular Photoluminescence

  ChemPhysChem · 2025-02-14 · 1 citations

  reviewOpen access1st author

  Abstract Attainment of quantum‐confined materials with remarkable stoichiometric, geometric, and structural control has been made possible by advances in colloidal nanoparticle synthesis. The quantum states of these systems can be tailored by selective spatial confinement in one, two, or three dimensions. As a result, a multitude of prospects for controlling nanoscale energy transfer have emerged. An understanding of the electronic relaxation dynamics for quantum states of specific nanostructures is required to develop predictive models for controlling energy on the nanoscale. Variable‐temperature, variable‐magnetic field ( ) optical methods have emerged as powerful tools for characterizing transient excited states. For example, magnetic circular photoluminescence (MCPL) spectroscopy can be used to calculate electronic g factors, assign spectroscopic term symbols for transitions within metal nanoclusters, and quantify the energy gaps separating electronic fine‐structure states. spectroscopic methods are effective for isolating the carrier dynamics of specific quantum fine‐structure states, enabling determination of electronic relaxation mechanisms such as electron‐phonon scattering and energy transfer between assembled nanoclusters. In particular ‐MCPL is especially effective for studying electronic spin‐state dynamics and properties. This Review highlights specific examples that emphasize insights obtainable from these methods and discusses prospects for future research directions.

  PublisherOA PDFDOI

Recent grants

• Understanding the Influence of Low-Frequency Vibrations on Energy Relaxation Through Layered Nanomaterials

  NSF · $376k · 2018–2023

• Collaborative Research: Excited State Dynamics of Structurally Precise Metal Nanoclusters

  NSF · $169k · 2017–2020

• Collaborative Research: Excited State Dynamics of Structurally Precise Metal Nanoclusters

  NSF · $320k · 2015–2018

• CAREER: Structure-specific Nanoscale Dynamics Studied by Nonlinear and Magneto-optical Spectroscopy

  NSF · $639k · 2012–2018

• CAREER: Structure-specific Nanoscale Dynamics Studied by Nonlinear and Magneto-optical Spectroscopy

  NSF · $64k · 2017–2019

Frequent coauthors

• Tian Zhao

  Peking University

  19 shared

• Jeremy W. Jarrett

  The University of Texas at Austin

  19 shared

• Chongyue Yi

  Florida State University

  18 shared

• Richard A. Vaia

  Wright-Patterson Air Force Base

  16 shared

• Julien Réhault

  University of Bern

  14 shared

• Christopher J. Ackerson

  Colorado State University

  12 shared

• Megan A. Steves

  University of California, Berkeley

  12 shared

• Patrick J. Herbert

  12 shared

Labs

• The Knappenberger GroupPI

Education

• Postdoc, Chemistry

  University of California Berkeley

  2008

• PhD, Chemistry

  Pennsylvania State University

  2005

Awards & honors

• Fellow of the American Association for the Advancement of Sc…
• Keynote Lecture, RACI National Centenary Conference, Royal A…
• Plenary Lecture, Photonics 2016, Optical Society Meeting, 20…
• Keynote Lecture, International Symposium on Molecular Spectr…
• Harold Kohn Endowed Alumni Lecturer, The Pennsylvania State…