Because
of the superior optical and electrical properties, copper-impregnated
size tuneable high-temperature stable manganese dioxide semiconductor
quantum dots (SQDs) have been successfully synthesized by a modified
chemical synthesis technique. Their size-dependent dielectric properties,
semiconducting properties, and current–voltage (I–V) characteristics have been investigated.
X-ray diffraction pattern and Raman spectra confirmed that the required
phase is present. Because of the different sintering temperature tuneable
size of SQDs has been found and confirmed by high-resolution transmission
electron microscopy. The band gap energy of the material is found
to be 1.25–1.67 eV, measured from Tauc plot using UV–vis
absorbance spectrum and their semiconducting properties have been
confirmed by the non linear current–voltage (I–V) behavior. Most intense green emission
peak of photoluminescence (PL) spectroscopy confirms the oxygen vacancy
defect state. The stoke shifting of Raman spectra, UV absorption,
and PL emission are the footprint of quantum confinement effect. Incorporation
of a little amount of Cu in tetragonal hollandite structure of α-MnO2 generates strain within that structure. This leads to create
sufficient crystal defect state as well as rise in dielectric constant
accompanied with low dielectric loss and higher ac conductivity. All
these highly desirable properties make the SQDs a potential candidate
for developing multifunctional photo-electronic devices. Owing to
the tuneable band gap and electronic transport of the SQDs, we realized
that the controllable size paves the way for designing SQDs possessing
unique properties for optical and electronic device applications.
Using this material as a high dielectric separator, a high-performance
supercapacitor has been successfully fabricated which can light up
15 light-emitting diodes for 47 min 23 s after charging them only
for 30 s.