His scientific works include novel energy materials and photovoltaic device concepts, carbon clusters in the Interstellar Medium, and intermolecular forces of real gases.

[6] In 2001/2002 he presented a bilayer heterojunction concept for organic photovoltaic devices applying vacuum processes for the formation of a molecular absorber layer consisting of Zn phthalocyanine and C60 molecules.

During his time as research fellow at the Institut für Medienkommunikation (IMK) of the former GMD-Forschungszentrum Informationstechnik [de] in Sankt Augustin near Bonn, Fostiropoulos' relationship to his colleague Simone Lahme, a web designer and specialist for e-learning, began on the occasion of the Total Solar eclipse of August 11, 1999, which they had been observing together in Karlsruhe.

In February 1989 he attended the "Dust Group" of Hugo Fechtig,[11] Department Cosmophysics at Max Planck Institute for Nuclear Physics, where he worked as doctoral candidate on his thesis "C60 – eine neue Form des Kohlenstoffs".

In September 1987 a young volunteering student, Bernd Wagner,[13] joined the "Dust Group" of astrophysicist Hugo Fechtig at Max Planck Institute for Nuclear Physics (MPIK) for a few weeks.

There, under the supervision of Fechtig's assistant Wolfgang Krätschmer Wagner had been performing such carbon vapour experiments using the contact arc method (2) when he accidentally changed the pressure conditions.

Applying a resistive heating process (3) and using such "isotopic" graphite rods, a few days before New Year's Eve 1989, he synthesized for the first time an unusual material which contained an exotic species: "isotopic" 13C60,[16] From that he proved the existence of an all-carbon molecule with icosahedral "platonic" symmetry and truncated icosahedron "archimedean" shape, i.e. that of Kroto et al. 1985 predicted football molecule: C60 "Buckminsterfullerene"[5] In his effort to push the production yield further he then focused his research on the simple arc method (1) that culminated in 1990 in a yield of > 1 gram per day[4] achieved by a robust and highly efficient arc process.

The extracted powder of this "natural" pure fullerene mixture was analyzed by optical (UV to IR) as well as mass spectroscopy where also the "American football" shaped C70 was found.

From the powder he could grow for the first time ultra-thin C60 films by vacuum thermal evaporation and watch the formation of hexagonal crystals from the "wine-red" benzene solution.

Beginning of 1991 Fostiropoulos had been invited to visit the research group of astrophysicist Alain Léger at the then Université Pierre-et-Marie-Curie, Paris-VI for two months where they performed matrix isolation spectroscopy on C−60 and C+60 ions in cold Ne matrices in order to measure their optical absorptions under quasi-space conditions.

[20] An immediate proposal in cooperation with Fostiropoulos submitted by Alain Léger and Louis Le Sergeant d'Hendecourt for one week observing time at the Canada–France–Hawaii Telescope[21] in order to search for C+60 fingerprints in the interstellar medium had been accepted.

With his results on the synthesis, extraction and characterization of fullerenes published 1990 in NATURE[23] (ResearchGate: 7,756 citations, as at October 2023) and some astrophysical aspects of ionized and hydrogenated C60 which he found in his data Fostiropoulos hastily completed his Ph.D. thesis by the end of 1991[24] that has been triggering waves of publications worldwide since then.

[23] In summer 1990 during a cooperation at the Heidelberg University on Raman spectroscopy to verify the hypothetical "football" structure of the molecule one of the partners expressed carefully his concerns: "hopefully it is not a cold fusion ;-)" referring to the most famous erratum at that time published by Martin Fleischmann et al. 1989.

[32] A few days later Harry Kroto, also Nobel Laureate (1996), and his wife Margaret unexpectedly visited the Heidelberg scientists on their way back from a conference in Yugoslavia.

In 1998 Fostiropoulos resumed scientific activities joining the Institute for Media Communication at the former German National Research Centre for Information Technology GMD-Forschungszentrum Informationstechnik [de] for three years where he participated in IT projects in the field of distance education.

He was awarded more than two million € mainly from the Federal Ministries BMU and BMBF, to a minor degree also by the Solar Energy Division at HMI to build and equip laboratories[37] for the development of vacuum preparation techniques for ultra-thin organic layer systems.

From 2014 on Fostiropoulos has been focussing on hybrid Perovskite solar cells whereupon his group developed an industrially relevant all-vacuum fabrication method for such devices.

During his early career (1989–1993) Fostiropoulos was granted three times fellowships from the Max Planck Society for his fundamental research on the synthesis of fullerenes and their characterization.

(For more information click "show" in collapsed table below) In the frame of the EU Coordination Action "OrgaPVNet"[39] he represented 20 German affiliates together with Andreas Hinsch, Fraunhofer Institute for Solar Energy Systems ISE.