Flame-formed carbon nanoparticles exhibit quantum dot behaviors

Abstract

We examine the quantum confinement in the photoemission ionization energy in air and optical band gap of carbon nanoparticles (CNPs). Premixed, stretched-stabilized ethylene flames are used to generate the CNPs reproducibly over the range of 4-23 nm in volume median diameter. The results reveal that flameformed CNPs behave like an indirect band gap material, and that the existence of the optical band gap is attributed to the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap in the polycyclic aromatic hydrocarbons comprising the CNPs. Both the ionization energy and optical band gap are found to follow closely the quantum confinement effect. The optical band gaps, measured both in situ and ex situ on the CNPs prepared in several additional flames, are consistent with the theory and the baseline data of CNPs from stretched-stabilized ethylene flames, thus indicating the observed effect to be general and that the particle size is the single most important factor governing the variation of the band gap of the CNPs studied. Cyclic voltammetry measurements and density functional theory calculations provide additional support for the quantum dot behavior observed. carbon nanoparticles | quantum dots | flames N anosized carbon grains composed of disordered polycyclic aromatic hydrocarbons (PAHs) are one of the most abundant forms of carbon material in nature (1). As a less-celebrated allotrope of carbon in comparison with fullerenes, nanotubes, and graphene, carbon nanoparticle (CNP) has its share of importance in a wide range of physical phenomena. Studies of interstellar emission spectra suggest that PAH dust grains or CNPs are ubiquitous in the interstellar media (2-5); and they are considered as the tracer of the star formation process (3, 6). CNPs are formed in flames during incomplete combustion. They are part of the soot-formation process. Mature soot or atmospheric black carbon particles often contain both elemental and organic carbon, the relative amounts of which vary by source (7). Soot impacts the global and region climate systems (8, 9) because of its light-absorbing properties (10, 11) and also as cloud condensation nuclei (12). CNPs 10-20 nm in size or fine carbon particles are themselves abundant in diesel exhausts (13). Their light-absorption properties and impact on climate forcing remain poorly characterized (14). As a material, however, CNPs have found their applications in photovoltaic and electrochemical devices, including perovskite solar cells in which flame CNPs act as a hole transfer medium (15).In contrast to the notion that soot, young or mature with respect to its growth in flames, is a broadband light absorber, recent studies have shown that CNPs several nanometers in size have well-defined optical band gaps or band edges (16)(17)(18). The band gap arises from the energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the constituent PAHs (18,19). While previous studies (16-18) explored the vari...

A Review of Terminology Used to Describe Soot Formation and Evolution under Combustion and Pyrolytic Conditions

“…73,242,387 Carbon nanoparticles (CNPs): The expression "carbon nanoparticles (CNPs)" is usually used to represent deliberately synthesized nanometer-scaled carbonaceous particles, including fullerenes, single-and multi-walled carbon nanotubes, and quantum dots. 82,[388][389][390][391][392] Aerosol: An aerosol is an ensemble of liquid and/or solid particles suspended in a gaseous medium, 92,305 whereas an aerosol particle is a single particle in an ensemble of suspended particles. 305 Smoke: Smoke is a suspension of combustion-generated particles that is concentrated enough to be visible to the naked eye.…”

Combustion in the future: The importance of chemistry

“…Flames are especially well suited to produce carbon nanomaterials with highly attractive properties [59] , [60] , [61] , [62] , [63] , [64] , [65] , [66] . The double-faced nature of soot as both, an air pollutant and a high-tech product has been recognized, and Mulay et al [59] have recently reviewed production and applications of candle soot.…”

“…Again, the facile, one-step method may show potential for tuning physico-chemical properties of the nanoparticles and thin films. As an exciting further aspect of carbon nanoparticles from flames, their quantum dot behavior has been recently investigated systematically [66] , with an analysis of their optical bandgap, photoemission ionization energy, and electrochemical ionization behavior. The findings of these authors and earlier research summarized in their report [66] regarding the light absorption and emission properties of carbon nanoparticles from flames will have impact not only for applications in photovoltaic or electrochemical devices, but also for assessing the effects of radiative forcing from combustion-generated soot.…”

Resistive Switching Phenomenon Observed in Self-Assembled Films of Flame-Formed Carbon-TiO2 Nanoparticles

“…They are produced in the highly reactive flame environment by a bottom-up process starting with the formation of polycyclic aromatic hydrocarbons [ 26 ], which are the building blocks of the nanoparticles. The particle properties and structure evolve in flame with the reaction time, and the fine control of the flame parameters allows the production of particles with desired size and optical, electrical and electronic properties [ 27 , 28 ]. Previous studies have shown that CNPs exhibit a quantum dot behavior [ 28 ], and their graphitization degree increases with particle residence time in flame [ 29 , 30 ] and flame temperature [ 31 , 32 ].…”