Method of depositing an atomic layer on a substrate, which method comprises - supplying a precursor gas from a precursor-gas supply comprised in a drum; the drum being rotatable with respect to a sealing piece that receives gas from a gas source; one of the drum or sealing piece comprising one or more gas gas feed channels in fluid connection with the precursor-gas supply; - the other of the drum or sealing piece comprises one or more circumferential grooves in its surface sealed by the one of the drum or sealing piece thereby preventing a fluid flowpath in radial direction and leaving a fluid flow path in circumferential direction; - during supplying the precursor gas from the precursor-gas supply towards the substrate the gas feed channels abut the sealed grooves wherein a part of the gas flow path is formed by the sealed grooves; - wherein at least one sealed groove is provided with one or more separations separating adjacent zones of process gas feeds in the sealed groove, thus allowing zones to provide for mutually differing process gas compositions; and - and further comprising providing relative motion between the precursor-gas supply and the substrate to have the precursor gas react near, e.g. on, the substrate so as to form an atomic layer by rotating the drum along a rotation trajectory while supplying the precursor gas; thus depositing a stack of atomic layers of a gradient composition, wherein the at least one groove is formed by an upstanding wall that is sealed by an abutting face of one of the sealing piece or drum; the separations being recessed relative to the upstanding wall thereby forming a flow restriction relative the abutting face in order to provide a controlled cross flow between adjacent zones. - supplying a precursor gas from a precursor-gas supply comprised in a drum; the drum being rotatable with respect to a sealing piece that receives gas from a gas source; one of the drum or sealing piece comprising one or more gas gas feed channels in fluid connection with the precursor-gas supply; - the other of the drum or sealing piece comprises one or more circumferential grooves in its surface sealed by the one of the drum or sealing piece thereby preventing a fluid flowpath in radial direction and leaving a fluid flow path in circumferential direction; - during supplying the precursor gas from the precursor-gas supply towards the substrate the gas feed channels abut the sealed grooves wherein a part of the gas flow path is formed by the sealed grooves; - wherein at least one sealed groove is provided with one or more separations separating adjacent zones of process gas feeds in the sealed groove, thus allowing zones to provide for mutually differing process gas compositions; and - and further comprising providing relative motion between the precursor-gas supply and the substrate to have the precursor gas react near, e.g. on, the substrate so as to form an atomic layer by rotating the drum along a rotation trajectory while supplying the precursor gas; thus depositing a stack of atomic layers of a gradient composition, wherein the at least one groove is formed by an upstanding wall that is sealed by an abutting face of one of the sealing piece or drum; the separations being recessed relative to the upstanding wall thereby forming a flow restriction relative the abutting face in order to provide a controlled cross flow between adjacent zones.