Band gap, refractive index, and reflectivity of InSb and PdSe₂ nanostructures vary with dimensionality

In a finding that could reshape how researchers design the next generation of optoelectronic devices, physicists at the University of Anbar in Iraq have demonstrated that the optical personality of two technologically important semiconductors, indium antimonide (InSb) and palladium diselenide (PdSe₂), can be tuned simply by changing the dimensionality of the structures they are built into. The study, published in Applied Nanoscience, provides a unified theoretical framework that connects the size and shape of quantum dots, nanowires, and nanosheets to their band gaps, refractive indices, and reflectivity—three properties that govern how materials absorb, transmit, and reflect light.

The research, carried out by Rasha Mushtaq Hashim and Saeed Naif Turki Al-Rashid of the Department of Physics, College of Education for Pure Science, tackles a long-standing challenge in nanoscience: predicting how optical properties change when a material is confined to fewer and fewer dimensions, without resorting to computationally expensive first-principles simulations. Instead, the team built their model around a quantity that is often overlooked in optical studies: cohesive energy, the energy that binds a solid’s atoms together.

The central insight of the work is that as a semiconductor structure shrinks, the fraction of its atoms sitting at the surface grows dramatically, weakening the average bonding in the crystal. This bond-order deficiency, a concept developed in earlier size-dependence literature, feeds directly into a cohesive-energy-based description of how the electronic band gap evolves with size. When the atoms at a surface have fewer neighbors to bond with, the electronic states near the band edges shift, and the band gap widens. The effect is the well-known phenomenon of quantum confinement, in which charge carriers—electrons and holes—are squeezed into a space comparable to or smaller than their natural extent in the bulk crystal, quantizing their allowed energies and pushing the lowest excitation energy upward.

Hashim and Al-Rashid applied this framework across three structural regimes. For zero-dimensional (0D) quantum dots, the relevant parameter is particle radius: smaller dots confine carriers more tightly, widening the gap. For one-dimensional (1D) nanowires and nanotubes, it is the diameter that controls the confinement energy. For two-dimensional (2D) layered nanosheets, the number of layers plays the analogous role—single layers behave very differently from multilayer stacks, and as layers accumulate the material gradually recovers its bulk character. In every case, the model predicts the same qualitative trend: the band gap decreases as the structure grows larger in its confined dimensions, converging toward the bulk value.

Crucially, the study does not stop at the band gap. Using the calculated size-dependent gap as an input, the researchers invoked the empirical Moss relation, a decades-old rule of thumb that links a semiconductor’s refractive index inversely to its band-gap energy, and then the Fresnel equations to compute reflectivity from the refractive index. The result is an elegant inversion of the band-gap trend: while the gap widens as structures shrink, the refractive index and reflectivity fall. Conversely, larger nanostructures—with weaker confinement—exhibit higher refractive indices and reflect more light. The physical logic is straightforward: a narrower band gap means the material’s electrons respond more readily to an oscillating electromagnetic field, increasing optical polarizability and hence the refractive index.

The two materials studied occupy fascinating and contrasting corners of the semiconductor landscape. InSb is a classic narrow-gap III–V semiconductor with a bulk band gap of only about 0.17 electronvolts, placing it squarely in the mid-infrared region of the spectrum. It has long been prized for infrared detectors, and its high-mobility charge carriers have made it a candidate material for next-generation electronics. PdSe₂, by contrast, belongs to the newer family of two-dimensional transition-metal dichalcogenides. Its puckered pentagonal layered structure sets it apart from the hexagonal geometry of graphene or molybdenum disulfide, and its air stability and widely tunable band gap—from around 1.3 electronvolts in monolayer form to a much smaller value in the bulk—have made it a magnet for research into photodetectors and layered electronics.

The model’s predictions capture these differences clearly. PdSe₂ generally exhibits larger band-gap values than InSb across the size ranges examined, consistent with its wider bulk gap and its strongly layer-dependent electronic structure. InSb, meanwhile, shows relatively higher refractive-index and reflectivity values in larger structures—a direct consequence of its narrow bulk band gap, which permits a large optical dielectric response. In practical terms, this means an InSb nanowire or thin film reflects and bends light more strongly than an equivalent PdSe₂ structure, a distinction that matters when these materials are integrated into photonic circuits, antireflection coatings, or infrared emitters.

The authors validated their approach against available experimental measurements and theoretical calculations from the literature, including photoluminescence studies of PdSe₂ quantum dots, layer-dependent optical measurements on chemical-vapor-deposition-grown PdSe₂ films, and experimental work on size-tuned InSb nanowires. The agreement was described as reasonable, particularly for band-gap trends across all three dimensionality regimes. Deviations in the predicted refractive indices and reflectivities were attributed to the limitations of the empirical Moss and Fresnel relations themselves, and to the model’s deliberate omission of wavelength dependence, crystal anisotropy, and surface-related effects—complications that are especially significant for PdSe₂, whose puckered structure is intrinsically anisotropic.

What makes the study notable is not the precision of individual numbers but the generality of the framework. Density functional theory calculations, while accurate, can require enormous computational resources when applied across many sizes, diameters, and layer counts. A cohesive-energy model, by contrast, needs only a handful of bulk material parameters and scales effortlessly across structural regimes. The authors had previously applied the same philosophy to predict size-dependent melting temperatures and Debye temperatures of noble-metal nanoparticles and the optical and thermal properties of CdSe and ZnSe nanoparticles, and the new work extends that program to a direct comparison of a narrow-gap III–V material and a layered dichalcogenide.

The practical implications reach into several hot areas of technology. Infrared photodetection, mid-infrared photonics, and quantum information science all demand semiconductors whose optical response is precisely engineered. InSb nanowires are actively explored as infrared sensors and as platforms for devices that exploit strong spin-orbit coupling. PdSe₂ nanosheets, with their tunable, layer-number-controlled gaps, are candidates for photodetectors spanning the visible to the infrared, for tunable optical coatings, and for van der Waals heterostructures in which different two-dimensional materials are stacked like atomic-scale LEGO bricks. A fast, inexpensive model that predicts how band gap, refractive index, and reflectivity evolve with dimensionality gives device designers a comparative shortcut before committing to synthesis or heavy simulation.

The study also serves as a reminder of a deeper principle in nanoscience: in the quantum world, geometry is destiny. The same chemical compound can behave as a wide-gap quantum emitter as a tiny dot, a moderate-gap wire, and a narrow-gap bulk absorber, with its reflectivity shifting in parallel. By capturing this geometry–optics relationship in a single cohesive-energy framework, Hashim and Al-Rashid have offered the nanoscience community a compact map of territory that would otherwise demand expedition after expedition with supercomputers.

The work, which received no external funding, was designed and simulated by Hashim, who developed the theoretical model and MATLAB implementation, with Al-Rashid supervising the theoretical framework and verifying the findings. The authors report no competing interests. As low-dimensional materials continue their march from laboratory curiosity to commercial device components, tools of this kind—simple, transparent, and physically interpretable—are likely to become fixtures in the optoelectronics design toolkit, guiding engineers toward the right material, in the right geometry, at the right size, for the optical job at hand.